Aaron J. Romanowsky
Expanded Bibliography
Condensed Bibliography |
Refereed Papers |
Unrefereed Papers |
Conference Proceedings
Refereed Papers
Constraining the stellar populations of ultra-diffuse galaxies in the MATLAS survey using spectral energy distribution fitting
M.L. Buzzo, D.A. Forbes, T.H. Jarrett, F.R. Marleau, P.-A. Duc, J.P. Brodie, A.J. Romanowsky, J.S. Gannon, S.R. Janssens, J. Pfeffer, A. Ferré-Mateu, L. Haacke, W.J. Couch, S. Lim, R. Sánchez-Janssen
Monthly Notices of the Royal Astronomical Society, Volume 529, Issue 4, 2024 April 21, pp. 3210–3234
ADS |
MNRAS |
arXiv:2402.12033
The star formation histories of quiescent ultra-diffuse galaxies and their dependence on environment and globular cluster richness
A. Ferré-Mateu, J.S. Gannon, D.A. Forbes, M.L. Buzzo, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 526, Issue 3, 2023 December 11, pp. 4735–4754
ADS |
MNRAS |
arXiv:2309.15148
Initial mass function variability from the integrated light of diverse stellar systems
C.M. Cheng, A. Villaume, M.L. Balogh, J.P. Brodie, I. Martín-Navarro, A.J. Romanowsky, P.G. van Dokkum
Monthly Notices of the Royal Astronomical Society, Volume 526, Issue 3, 2023 December 11, pp. 4004–4023
ADS |
MNRAS |
arXiv:2309.14415
Keck spectroscopy of NGC 1052-DF9: stellar populations in the context of the NGC 1052 group
J.S. Gannon, M.L. Buzzo, A. Ferré-Mateu, D.A. Forbes, J.P. Brodie, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 524, Issue 2, 2023 September 11, pp. 2624–2629
ADS |
MNRAS |
arXiv:2306.08540
The large-scale structure of globular clusters in the NGC 1052 group
M.L. Buzzo, D.A. Forbes, J.P. Brodie, S.R. Janssens, W.J. Couch, A.J. Romanowsky, J.S. Gannon
Monthly Notices of the Royal Astronomical Society, Volume 522, Issue 1, 2023 June 11, pp. 595–605
ADS |
MNRAS |
arXiv:2303.16375
A candidate runaway supermassive black hole identified by shocks and star formation in its wake
P. van Dokkum, I. Pasha, M.L. Buzzo, S. LaMassa, Z. Shen, M.A. Keim, R. Abraham, C. Conroy, S. Danieli, K. Mitra, D. Nagai, P. Natarajan, A.J. Romanowsky, G. Tremblay, C.M. Urry, F.C. van den Bosch
The Astrophysical Journal Letters, Volume 946, Number 2, 2023 April 1, Article L50 (14pp)
ADS |
ApJL |
arXiv:2302.04888
Reconstructing the genesis of a globular cluster system at a look-back time of 9.1 Gyr with the JWST
D.A. Forbes, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 518, Issue 3, 2023 January 21, pp. 3653–3666
ADS |
MNRAS |
arXiv:2212.05078
Keck spectroscopy of the coma cluster ultra-diffuse galaxy Y358: dynamical mass in a wider context
J.S. Gannon, D.A. Forbes, J.P. Brodie, A.J. Romanowsky, W.J. Couch, A. Ferré-Mateu
Monthly Notices of the Royal Astronomical Society, Volume 518, Issue 3, 2023 January 21, pp. 3653–3666
ADS |
MNRAS |
arXiv:2211.03915
Low-density star cluster formation: discovery of a young faint fuzzy on the outskirts of the low-mass spiral galaxy NGC 247
A.J. Romanowsky, S.S. Larsen, A. Villaume, J.L. Carlin, J. Janz, D.J. Sand, J. Strader, J.P. Brodie, S. Chakrabarti, C.M. Cheng, D. Crnojević, D.A. Forbes, C.T. Garling, J.R. Hargis, A. Karunakaran, I. Martín-Navarro, K.A.G. Olsen, N. Rider, B. Salimkumar, V. Santhanakrishnan, K. Spekkens, Y. Tang, P.G. van Dokkum, B. Willman
Monthly Notices of the Royal Astronomical Society, Volume 518, Issue 2, 2023 January 11, pp. 3164–3182
ADS |
MNRAS |
arXiv:2210.03220
Monochromatic globular clusters as a critical test of formation models for the dark matter-deficient galaxies NGC 1052-DF2 and NGC 1052-DF4
P. van Dokkum, Z. Shen, A.J. Romanowsky, R. Abraham, C. Conroy, S. Danieli, D. Dutta Chowdhury, M.A. Keim, J.M.D. Kruijssen, J. Leja, S. Trujillo-Gomez
The Astrophysical Journal Letters, Volume 940, Number 1, 2022 November 20, Article L9 (9pp)
ADS |
ApJL |
arXiv:2207.07129
The stellar populations of quiescent ultra-diffuse galaxies from optical to mid-infrared spectral energy distribution fitting
M.L. Buzzo, D.A. Forbes, J.P. Brodie, A.J. Romanowsky, M.E. Cluver, T.H. Jarrett, S. Laine, W.J. Couch, J.S. Gannon, A. Ferré-Mateu, N. Okabe
Monthly Notices of the Royal Astronomical Society, Volume 517, Issue 2, 2022 December 1, pp. 2231–2250
ADS |
MNRAS |
arXiv2208:11819
The globular clusters and star formation history of the isolated, quiescent ultra-diffuse galaxy DGSAT I
S.R. Janssens, A.J. Romanowsky, R. Abraham, J.P. Brodie, W.J. Couch, D.A. Forbes, S. Laine, D. Martínez-Delgado, P.G. van Dokkum
Monthly Notices of the Royal Astronomical Society, Volume 517, Issue 1, 2022 November 21, pp. 858–871
ADS |
MNRAS |
arXiV:2209.09910
Still at odds with conventional galaxy evolution: the star formation history of ultradiffuse galaxy Dragonfly 44
K.A. Webb, A. Villaume, S. Laine, A.J. Romanowsky, M. Balogh, P. van Dokkum, D.A. Forbes, J. Brodie, C. Martin, M. Matuszewski
Monthly Notices of the Royal Astronomical Society, Volume 516, Issue 3, 2022 November 1, pp. 3318–3341
ADS |
MNRAS |
arXiv:2208.11038
Light from the darkness: detecting ultra-diffuse galaxies in the Perseus cluster through over-densities of globular clusters with a log-Gaussian Cox process
D.D. Li, G.M. Eadie, R. Abraham, P.E. Brown, W.E. Harris, S.R. Janssens, A.J. Romanowsky, P. van Dokkum, S. Daneli
The Astrophysical Journal, Volume 935, Number 1, 2022 August 10, Article 3 (28pp)
ADS |
ApJ |
arXiv:2204.05487
The halo of M 105 and its group environment as traced by planetary nebula populations. II. Using kinematics of single stars to unveil the presence of intragroup light around the Leo I galaxies NGC 3384 and M 105
J. Hartke, M. Arnaboldi, O. Gerhard, L. Coccato, M. Merrifield, K. Kuijken, C. Pulsoni, A. Agnello, S. Bhattacharya, C. Spiniello, A. Cortesi, K.C. Freeman, N.R. Napolitano, A.J. Romanowsky
Astronomy & Astrophysics, Volume 663, 2022 July 1, Article A12 (19pp)
ADS |
A&A |
arXiv:2201.08710
A trail of dark-matter-free galaxies from a bullet-dwarf collision
P. van Dokkum, Z. Shen, M.A. Keim, S. Trujillo-Gomez, S. Danieli, D. Dutta Chowdhury, R. Abraham, C. Conroy, J.M.D. Kruijssen, D. Nagai, A. Romanowsky
Nature, Volume 605, Issue 7910, 2022 May 18, pp. 435–439
ADS |
Nature |
arXiv:2205.08552 |
Low-metallicity globular clusters in the low-mass isolated spiral galaxy NGC 2403
D.A. Forbes, A. Ferré-Mateu, J.S. Gannon, A.J. Romanowsky, J.L. Carlin, J.P. Brodie, J. Day
Monthly Notices of the Royal Astronomical Society, Volume 512, Issue 1, 2022 May 1, pp. 802–810
ADS |
MNRAS |
arXiv:2202.10539
The chemical composition of globular clusters in the Local Group
S.S. Larsen, P. Eitner, E. Magg, M. Bergemann, C.A.S. Moltzer, J.P. Brodie, A.J. Romanowsky, J. Strader
Astronomy & Astrophysics, Volume 660, April 2022, Article A88 (46pp)
ADS |
A&A |
arXiv:2112.00081
NGC 5846-UDG1: a galaxy formed mostly by star formation in massive, extremely dense clumps of gas
S. Danieli, P. van Dokkum, S. Trujillo-Gomez, J.M.D. Kruijssen, A.J. Romanowsky, S. Carlsten, Z. Shen, J. Li, R. Abraham, J. Brodie, C. Conroy, J.S. Gannon, J. Greco
The Astrophysical Journal Letters, Volume 927, Number 2, 2022 March 10, Article L28 (9pp)
ADS |
ApJL |
arXiv:2111.14851
The present-day globular cluster kinematics of lenticular galaxies from the E-MOSAICS simulations and their relation to the galaxy assembly histories
A. Dolfi, J. Pfeffer, D.A. Forbes, W.J. Couch, K. Bekki, J.P. Brodie, A.J. Romanowsky, J.M.D. Kruijssen
Monthly Notices of the Royal Astronomical Society, Volume 511, Issue 3, 2022 April 11, pp. 3179–3197
ADS |
MNRAS |
arXiv:2202.00020
Spatially resolved stellar spectroscopy of the ultra-diffuse galaxy Dragonfly 44. III. Evidence for an unexpected star formation history under conventional galaxy evolution processes
A. Villaume, A.J. Romanowsky, J. Brodie, P. van Dokkum, C. Conroy, D.A. Forbes, S. Danieli, C. Martin, M. Matuszewski
The Astrophysical Journal, Volume 924, Number 1, 2022 January 1, Article 32 (13pp)
ADS |
ApJ |
arXiv:2101.02220
Ultra-diffuse galaxies in the perseus cluster: comparing galaxy properties with globular cluster system richness
J.S. Gannon, D.A. Forbes, A.J. Romanowsky, A. Ferré-Mateu, W.J. Couch, J.P. Brodie, S. Huang, S.R. Janssens, N. Okabe
Monthly Notices of the Royal Astronomical Society, Volume 510, Issue 1, 2022 February 11, pp. 946–958
ADS |
MNRAS |
arXiv:2111.06007
Hubble Space Telescope imaging of the extremely metal-poor globular cluster EXT8 in Messier 31
S.S. Larsen, A.J. Romanowsky, J.P. Brodie
Astronomy & Astrophysics, Volume 651, July 2021, Article A102 (19pp)
A&A |
ADS |
arXiv:2105.01674
A recently quenched isolated dwarf galaxy outside of the Local Group environment
A. Polzin, P. van Dokkum, S. Danieli, J.P. Greco, A.J. Romanowsky
The Astrophysical Journal Letters, Volume 914, Number 1, 2021 June 10, Article L23 (7pp)
ADS |
ApJL |
arXiv:2105.08061
A tip of the red giant branch distance of 22.1 ± 1.2 Mpc to the dark matter deficient galaxy NGC 1052–DF2 from 40 orbits of Hubble Space Telescope imaging
Z. Shen, S. Danieli, P. van Dokkum, R. Abraham, J.P. Brodie, C. Conroy, A.E. Dolphin, A.J. Romanowsky, J.M.D. Kruijssen, D. Dutta Chowdhury
The Astrophysical Journal Letters, Volume 914, Number 1, 2021 June 10, Article L12 (9pp)
ADS |
ApJL |
arXiv:2104.03319
The SLUGGS survey: combining stars, globular clusters, and planetary nebulae to understand the assembly history of early-type galaxies from their large radii kinematics
A. Dolfi, D.A. Forbes, W.J. Couch, K. Bekki, A. Ferré-Mateu, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 504, Issue 4, 2021 July 11, pp. 4923–4939
ADS |
MNRAS |
arXiv:2104.06396
Recovering the origins of the lenticular galaxy NGC 3115 using multiband imaging
M.L. Buzzo, A. Cortesi, J.A. Hernandez-Jimenez, L. Coccato, A. Werle, L. Beraldo e Silva, M. Grossi, M. Vika, C.E. Barbosa, G. Lucatelli, L. Santana-Silva, S. Bamford, V.P. Debattista, D.A. Forbes, R. Overzier, A.J. Romanowsky, F. Ferrari, J.P. Brodie, C. Mendes de Oliveira
Monthly Notices of the Royal Astronomical Society, Volume 504, Issue 2, 2021 June 21, pp. 2146–2167
ADS |
MNRAS |
arXiv:2103.17246
Low-mass compact elliptical galaxies: spatially resolved stellar populations and kinematics with the Keck Cosmic Web Imager
A. Ferré-Mateu, M. Durré, D.A. Forbes, A.J. Romanowsky, A. Alabi, J.P. Brodie, R.M. McDermid
Monthly Notices of the Royal Astronomical Society, Volume 503, Issue 4, 2021 June 1, pp. 5455–5472
ADS |
MNRAS |
arXiv:2103.09241
Hubble Space Telescope observations of two faint dwarf satellites of nearby LMC analogs from MADCASH
J.L. Carlin, B. Mutlu-Pakdil, D. Crnojević, C.T. Garling, A. Karunakaran, A.H.G. Peter, E. Tollerud, D.A. Forbes, J.R. Hargis, S. Lim, A.J. Romanowsky, D.J. Sand, K. Spekkens, J. Strader
The Astrophysical Journal, Volume 909, Number 2, 2021 March 10, Article 211 (16pp)
ADS |
ApJ |
arXiv:2012.09174
Stellar velocity dispersion and dynamical mass of the ultra-diffuse galaxy NGC 5846_UDG1 from the Keck Cosmic Web Imager
D.A. Forbes, J.S. Gannon, A.J. Romanowsky, A. Alabi, J.P. Brodie, W.J. Couch, A. Ferré-Mateu
Monthly Notices of the Royal Astronomical Society, Volume 500, Issue 1, 2021 January 1, pp. 1279–1284
ADS |
MNRAS |
arXiv:2010.07313
An extremely metal-deficient globular cluster in the Andromeda Galaxy
S.S. Larsen, A.J. Romanowsky, J.P. Brodie, A. Wasserman
Science, Volume 370, Issue 6519, 20 November 2020, pp. 970–973
ADS |
Science |
arXiv:2010.07395
Press coverage:
Keck |
NOVA |
SJSU |
Swinburne |
ScienceNews |
EarthSky |
Wikipedia
The assembly history of M87 through radial variations in chemical abundances of its field star and globular cluster populations
A. Villaume, D. Foreman-Mackey, A.J. Romanowsky, J. Brodie, J. Strader
The Astrophysical Journal, Volume 900, Number 2, 2020 September 10, Article 95 (19pp)
ADS |
ApJ |
arXiv:2006.16280
Keck Cosmic Web Imager (KCWI) spectra of globular clusters and ultracompact dwarfs in the halo of M87
D.A. Forbes, A. Ferré-Mateu, M. Durré, J.P. Brodie, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 497, Issue 1, 2020 September 1, pp. 765–775
ADS |
MNRAS |
arXiv:2007.00739
NGC 474 as viewed with KCWI: diagnosing a shell galaxy
A.B. Alabi, A. Ferré-Mateu, D.A. Forbes, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 497, Issue 1, 2020 September 1, pp. 626–631
ADS |
MNRAS |
arXiv:2007.01870
An expanded catalogue of low surface brightness galaxies in the Coma cluster using Subaru/Suprime-Cam
A.B. Alabi, A.J. Romanowsky, D.A. Forbes, J.P. Brodie, N. Okabe
Monthly Notices of the Royal Astronomical Society, Volume 496, Issue 3, 2020 August 11, pp. 3182–3197
ADS |
MNRAS |
arXiv:2006.10043
On the stellar kinematics and mass of the Virgo ultradiffuse galaxy VCC 1287
J.S. Gannon, D.A. Forbes, A.J. Romanowsky, A. Ferré-Mateu, W.J. Couch, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 495, Issue 3, 2020 July 1, pp. 2582–2598
ADS |
MNRAS |
arXiv:2005.03041
The assembly history of the nearest S0 galaxy NGC 3115 from its kinematics out to six half-light radii
A. Dolfi, D.A. Forbes, W.J. Couch, A. Ferré-Mateu, S. Bellstedt, K. Bekki, J. Diaz, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 495, Issue 1, 2020 June 11, pp. 1321–1339
ADS |
MNRAS |
arXiv:2004.09818
A tip of the red giant branch distance to the dark matter deficient galaxy NGC 1052-DF4 from deep Hubble Space Telescope data
S. Danieli, P. van Dokkum, R. Abraham, C. Conroy, A.E. Dolphin, A.J. Romanowsky
The Astrophysical Journal Letters, Volume 895, Number 1, 2020 May 20, Article L4 (8pp)
ADS |
ApJL |
arXiv:1910.07529
Press coverage:
Astronomy |
astrobites |
Forbes |
Discover
Multiwavelength follow-up of the hyperluminous intermediate-mass black hole candidate 3XMM J215022.4–055108
D. Lin, J. Strader, A.J. Romanowsky, J.A. Irwin, O. Godet, D. Barret, N.A. Webb, J. Homan, R.A. Remillard
The Astrophysical Journal Letters, Volume 892, Number 2, 2020 April 1, Article L25 (5pp)
ADS |
ApJL |
arXiv:2002.04618
Press coverage:
HST/ESA |
NASA |
HST |
AAS Nova |
SyfyWire/Bad Astronomy
The PIPER Survey: I. An initial look at the intergalactic globular cluster population in the Perseus Cluster
W.E. Harris, R.A. Brown, P.R. Durrell, A.J. Romanowsky, J. Blakeslee, J. Brodie, S. Janssens, T. Lisker, S. Okamoto, C. Wittmann
The Astrophysical Journal, Volume 890, Number 2, 2020 February 20, Article 105 (14pp)
ADS |
ApJ |
arXiv:2001.06391
Globular clusters in Coma cluster ultra-diffuse galaxies (UDGs): evidence for two types of UDG?
D.A. Forbes, A. Alabi, A.J. Romanowsky, J.P. Brodie, N. Arimoto
Monthly Notices of the Royal Astronomical Society, Volume 492, Issue 4, 2020 March 11, pp. 4874–4883
ADS |
MNRAS |
arXiv:2001.10031
Globular clusters in the stellar stream surrounding the Milky Way analogue NGC 5907
A B. Alabi, D.A. Forbes, A.J. Romanowsky, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 491, Issue 4, 2020 February 1, pp. 5693–5701
ADS |
MNRAS |
arXiv:1912.00999
The distribution of ultra-diffuse and ultra-compact galaxies in the Frontier Fields
S.R. Janssens, R. Abraham, J. Brodie, D.A. Forbes, A.J. Romanowsky
The Astrophysical Journal, Volume 887, Number 1, 2019 December 10, Article 92 (19pp)
ADS |
ApJ |
arXiv:1911.00011
Tidal destruction in a low-mass galaxy environment: the discovery of tidal tails around DDO 44
J.L. Carlin, C.T. Garling, A.H.G. Peter, D. Crnojević, D.A. Forbes, J.R. Hargis, B. Mutlu-Pakdil, R. Pucha, A.J. Romanowsky, D.J. Sand, K. Spekkens, J. Strader, B. Willman
The Astrophysical Journal, Volume 886, Number 2, 2019 December 1, Article 109 (11pp)
ADS |
ApJ |
arXiv:1906.08260
Spatially-resolved stellar kinematics of the ultra-diffuse galaxy Dragonfly 44. II. Constraints on fuzzy dark matter
A. Wasserman, P. van Dokkum, A.J. Romanowsky, J. Brodie, S. Danieli, D.A. Forbes, R. Abraham, C. Martin, M. Matuszewski, A. Villaume, J. Tamanas, S. Profumo
The Astrophysical Journal, Volume 885, Number 2, 2019 November 10, Article 155 (11pp)
ADS |
ApJ |
arXiv:1905.10373
The SLUGGS survey: measuring globular cluster ages using both photometry and spectroscopy
C. Usher, J.P. Brodie, D.A. Forbes, A.J. Romanowsky, J. Strader, J. Pfeffer, N. Bastian
Monthly Notices of the Royal Astronomical Society, Volume 490, Issue 1, 2019 November 21, pp. 491–501
ADS |
MNRAS |
arXiv:1909.05753
Dark matter and no dark matter: on the halo mass of NGC 1052
D.A. Forbes, A. Alabi, J.P. Brodie, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 489, Issue 3, 2019 November 1, pp. 3665–3669
ADS |
MNRAS |
arXiv:1908.10858
Hyper wide field imaging of the Local Group dwarf irregular galaxy IC 1613: an extended component of metal-poor stars
R. Pucha, J.L. Carlin, B. Willman, J. Strader, D.J. Sand, K. Bechtol, J.P. Brodie, D. Crnojević, D.A. Forbes,
C. Garling, J. Hargis, A.H.G. Peter, A.J. Romanowsky
The Astrophysical Journal, Volume 880, Number 2, 2019 August 1, Article 104 (11pp)
ADS |
ApJ |
arXiv:1905.02210
Spatially-resolved stellar kinematics of the ultra-diffuse galaxy Dragonfly 44. I. Observations, kinematics, and cold dark matter halo fits
P. van Dokkum, A. Wasserman, S. Danieli, R. Abraham, J. Brodie, C. Conroy, D.A. Forbes, C. Martin, M. Matuszewski, A.J. Romanowsky, A. Villaume
The Astrophysical Journal, Volume 880, Number 2, 2019 August 1, Article 91 (26pp)
ADS |
ApJ |
arXiv:1904.04838
New constraints on early-type galaxy assembly from spectroscopic metallicities of globular clusters in M87
A. Villaume, A.J. Romanowsky, J. Brodie, J. Strader
The Astrophysical Journal, Volume 879, Number 1, 2019 July 1, Article 45 (13pp)
ADS |
ApJ |
arXiv:1904.12641
The observed characteristics of globular cluster (GC) systems, such as metallicity
distributions, are commonly used to place constraints on galaxy formation models. However,
obtaining reliable metallicity values is particularly difficult because of our limited means to
obtain high quality spectroscopy of extragalactic GCs. Often, "color–metallicity
relations" are invoked to convert easier-to-obtain photometric measurements into metallicities,
but there is no consensus on what form these relations should take. In this paper we make use of
multiple photometric data sets and iron metallicity values derived from applying full-spectrum
stellar population synthesis models to deep Keck/LRIS spectra of 177 GCs centrally located
around M87 to obtain a new color–metallicity relation. Our new relation differs
substantially from previous relations in the blue, and we present evidence that the M87 relation
differs from that of the Milky Way GCs, suggesting environmental dependence of GC properties. We
use our color–metallicity relation to derive a new GC metallicity-host galaxy luminosity
relation for red and blue GCs and find a shallower relation for the blue GCs than what previous
work has found and that the metal-poor GCs are more enriched than what was previously found.
This could indicate that the progenitor satellite galaxies that now make up the stellar halos of
early-type galaxies are more massive and formed later than previously thought, or that the
properties of metal-poor GCs are less dependent on their present-day host, indicating a common
origin.
Formation of ultra-diffuse galaxies in the field and in galaxy groups
F. Jiang, A. Dekel, J. Freundlich, A.J. Romanowsky, A.A. Dutton, A.V. Macciò, A. Di Cintio
Monthly Notices of the Royal Astronomical Society, Volume 487, Issue 4, 2019 August 11, pp. 5272–5290
ADS |
MNRAS |
arXiv:1811.10607
We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking
the origin of UDGs in the field versus galaxy groups. We find that while field UDGs
arise from dwarfs in a characteristic mass range by multiple episodes of supernova
feedback (Di Cintio et al.), group UDGs may also form by tidal puffing up and they
become quiescent by ram-pressure stripping. The field and group UDGs share similar
properties, independent of distance from the group centre. Their dark-matter haloes
have ordinary spin parameters and centrally dominant dark-matter cores. Their
stellar components tend to have a prolate shape with a Sérsic index
n ~ 1 but no significant rotation. Ram pressure removes the gas from the
group UDGs when they are at pericentre, quenching star formation in them and making
them redder. This generates a colour/star-formation-rate gradient with distance from
the centre of the dense environment, as observed in clusters. We find that ~20
per cent of the field UDGs that fall into a massive halo survive as satellite UDGs.
In addition, normal field dwarfs on highly eccentric orbits can become UDGs near
pericentre due to tidal puffing up, contributing about half of the group-UDG
population. We interpret our findings using simple toy models, showing that gas
stripping is mostly due to ram pressure rather than tides. We estimate that the
energy deposited by tides in the bound component of a satellite over one orbit can
cause significant puffing up provided that the orbit is sufficiently eccentric. We
caution that while the simulations produce UDGs that match the observations, they
under-produce the more compact dwarfs in the same mass range, possibly because of
the high threshold for star formation or the strong feedback.
Spatially resolved stellar populations and kinematics with KCWI: probing the assembly history of the massive early-type galaxy NGC 1407
A. Ferré-Mateu, D.A. Forbes, R. McDermid, A.J. Romanowsky, J.P. Brodie
The Astrophysical Journal, Volume 878, Number 2, 2019 June 20, Article 129 (9pp)
ADS |
ApJ |
arXiv:1905.08818
Using the newly commissioned Keck Cosmic Web Imager (KCWI) instrument on the Keck II
telescope, we analyze the stellar kinematics and stellar populations of the
well-studied massive early-type galaxy (ETG) NGC 1407. We obtained high
signal-to-noise integral field spectra for a central and an outer (around one
effective radius toward the southeast direction) pointing with integration times of
just 600 s and 2400 s, respectively. We confirm the presence of a kinematically
distinct core also revealed by VLT/MUSE data of the central regions. While NGC 1407
was previously found to have stellar populations characteristic of massive ETGs
(with radially constant old ages and high alpha-enhancements), it was claimed to
show peculiar supersolar metallicity peaks at a large radius that deviated from an
otherwise strong negative metallicity gradient, which is hard to reconcile within a
"two-phase" formation scenario. Our outer pointing confirms the near-uniform old
ages and the presence of a steep metallicity gradient, but with no evidence for
anomalously high metallicity values at large galactocentric radii. We find a rising
outer velocity dispersion profile and high values of the fourth-order kinematic
moment—an indicator of possible anisotropy. This coincides with the reported
transition from a bottom-heavy to a Salpeter initial mass function, which may
indicate that we are probing the transition region from the "in situ" to the
accreted phase. With short exposures, we have been able to derive robust stellar
kinematics and stellar populations in NGC 1407 to ~1 effective radius. This
experiment shows that future work with KCWI will enable 2D kinematics and stellar
populations to be probed within the low surface brightness regions of galaxy halos
in an effective way.
Still missing dark matter: KCWI high-resolution stellar kinematics of NGC1052-DF2
S. Danieli, P. van Dokkum, C. Conroy, R. Abraham, A.J. Romanowsky
The Astrophysical Journal Letters, Volume 874, Number 2, 2019 April 1, Article L12 (8pp)
ADS |
ApJL |
arXiv:1901.03711
Press coverage:
Keck |
Forbes |
Scientific American
The velocity dispersion of the ultra diffuse galaxy NGC1052-DF2 was found to be
σgc = 7.8+5.2–2.2 km s–1,
much lower than expected from the stellar mass–halo mass relation and nearly identical
to the expected value from the stellar mass alone. This result was based on the radial
velocities of 10 luminous globular clusters that were assumed to be associated with the
galaxy. A more precise measurement is possible from high-resolution spectroscopy of the
diffuse stellar light. Here we present an integrated spectrum of the diffuse light of
NGC1052-DF2 obtained with the Keck Cosmic Web Imager (KCWI), with an instrumental
resolution of σinstr ≈ 12 km s–1. The
systemic velocity of the galaxy is vsys = 1805 ± 1.1 km s–1,
in very good agreement with the average velocity of the globular clusters
(<vgc> = 1803 ± 2 km s–1). There is no evidence for
rotation within the KCWI field of view. We find a stellar velocity dispersion of
σstars = 8.5+2.3–3.1 km s–1,
consistent with the dispersion that was derived from the globular clusters. The implied
dynamical mass within the half-light radius r1/2 = 2.7 kpc is
Mdyn = (1.3 ± 0.8) × 108 M☉,
similar to the stellar mass within that radius (Mstars = (1.0 ± 0.2)
× 108 M☉). With this confirmation of the low
velocity dispersion of NGC1052-DF2, the most urgent question is whether this "missing dark
matter problem" is unique to this galaxy or applies more widely.
A second galaxy missing dark matter in the NGC 1052 group
P. van Dokkum, S. Danieli, R. Abraham, C. Conroy, A.J. Romanowsky
The Astrophysical Journal Letters, Volume 874, Number 1, 2019 March 20, Article L5 (8pp)
ADS |
ApJL |
arXiv:1901.05973
Press coverage:
Keck |
Scientific American |
Sky & Telescope
The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that
it has little or no dark matter. Here we report the discovery of a second galaxy in this
class, residing in the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of
its size, surface brightness, and morphology; has a similar distance of
Dsbf = 19.9 ± 2.8 Mpc; and also has a population of luminous
globular clusters extending out to ≥7 kpc from the center of the galaxy. Accurate
radial velocities of the diffuse galaxy light and seven of the globular clusters were
obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The
velocity of the diffuse light is identical to the median velocity of the clusters,
vsys = <vgc> = 1445 km s–1, and close
to the central velocity of the NGC 1052 group. The rms spread of the globular cluster
velocities is very small at σobs = 5.8 km s–1.
Taking observational uncertainties into account we determine an intrinsic velocity
dispersion of σintr = 4.2+4.4–2.2
km s–1, consistent with the expected value from the stars alone
(σstars ≈ 7 km s–1) and lower than expected
from a standard NFW halo (σhalo ~ 30 km s–1).
We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects
exists. The origin of these large, faint galaxies with an excess of luminous globular
clusters and an apparent lack of dark matter is, at present, not understood.
Extreme chemical abundance ratio suggesting an exotic origin for an ultradiffuse galaxy
I. Martín-Navarro, A.J. Romanowsky, J.P. Brodie, A. Ferré-Mateu,
A. Alabi, D.A. Forbes, M. Sharina, A. Villaume, V. Pandya, D. Martinez-Delgado
Monthly Notices of the Royal Astronomical Society, Volume 484, Issue 3, 2019 April 11, pp. 3425–3433
ADS |
MNRAS |
arXiv:1901.08068
Press coverage:
Keck |
Forbes
Ultradiffuse galaxies (UDGs) are a population of extended galaxies but with relatively low
luminosities. The origin of these objects remains unclear, largely due to the observational
challenges of the low surface brightness Universe. We present here a detailed stellar
population analysis of a relatively isolated UDG, DGSAT I, based on spectroscopic data from
the Keck Cosmic Web Imager integral field unit. The star formation history of DGSAT I seems
to be extended, with a mean luminosity-weighted age of ~3 Gyr, in agreement with previous
photometric studies. However, we find a very high [Mg/Fe] abundance ratio, which is extreme
even in the context of the highly alpha-enhanced massive ellipticals and ultrafaint dwarfs.
The [Mg/Fe] enhancement of DGSAT I appears to be 10 times higher than the most
magnesium-enhanced stellar systems discovered to date, and suggests that the chemical
enrichment of this object was dominated by core-collapse supernovae. Intriguingly, this
breaks the canonical relation between [Mg/Fe] and star formation time-scale. With a measured
velocity dispersion of 56 ± 10 km s–1, DGSAT I also shows a
high-mass-to-light ratio, which indicates that it is highly dark matter dominated. The
metal-poor conditions of DGSAT I may have enhanced the formation of massive stars, while at
the same time, additional mechanisms are needed to prevent iron-rich yields from being
recycled into stars. These results suggest that some UDGs could have experienced chemical
enrichment episodes similar to the first building blocks of galaxies.
Mirach's Goblin:
Discovery of a dwarf spheroidal galaxy behind the Andromeda galaxy
D. Martínez-Delgado, E.K. Grebel, B. Javanmardi, W. Boschin, N. Longeard, J.A. Carballo-Bello, D. Makarov, M.A. Beasley, G. Donatiello, M.P. Haynes, D.A. Forbes, A.J. Romanowsky
Astronomy & Astrophysics, Volume 620, December 2018, Article A126 (10pp)
ADS |
A&A |
arXiv:1810.04741
It is of broad interest for galaxy formation theory to carry out a full inventory of the
numbers and properties of dwarf galaxies, both satellite and isolated, in the Local
Volume. Ultra-deep imaging in wide areas of the sky with small amateur telescopes can
help to complete the census of these hitherto unknown low-surface-brightness galaxies,
which cannot be detected by the current resolved stellar population and HI surveys. We
report the discovery of Donatiello I, a dwarf spheroidal galaxy located one degree from
the star Mirach (β And) in a deep image taken with an amateur telescope.
The color-magnitude diagram (CMD) obtained from follow-up observations obtained with the
Gran Telescopio Canarias (La Palma, Spain) reveals that this system is beyond the local
group and is mainly composed of old stars. The absence of young stars and HI emission in
the ALFALFA survey is typical of quenched dwarf galaxies. Our photometry suggests a
distance modulus for this galaxy of (m–M) = 27.6 ± 0.2 (3.3 Mpc),
although this distance cannot yet be established securely owing to the crowding effects
in our CMD. At this distance, the absolute magnitude (MV = –8.3),
surface brightness (μV = 26.5 mag arcsec–2), and
stellar content of Donatiello I are similar to the "classical" Milky Way companions
Draco or Ursa Minor.
The projected position and distance of Donatiello I are consistent with this object
being a dwarf satellite of the closest S0-type galaxy NGC 404 ("Mirach's Ghost").
Alternatively, it could be one of the most isolated quenched dwarf galaxies reported so
far behind the Andromeda galaxy.
Angular momentum and galaxy formation revisited: scaling relations for disks and bulges
S.M. Fall, A.J. Romanowsky
The Astrophysical Journal, Volume 868, Number 2, 2018 December 1, Article 133 (13pp)
ADS |
ApJ |
arXiv:1808.02525
We show that the stellar specific angular momentum j⭑, mass
M⭑, and bulge fraction β⭑ of
normal galaxies of all morphological types are consistent with a simple model based
on a linear superposition of independent disks and bulges. In this model, disks and
bulges follow scaling relations of the form j⭑d ∝
M⭑dα and
j⭑b ∝
M⭑bα with α =
0.67 ± 0.07 but offset from each other by a factor of 8 ± 2 over the mass
range 8.9 ≤ log(M⭑/M☉)
≤ 11.8. Separate fits for disks and bulges alone give α =
0.58 ± 0.10 and α = 0.83 ± 0.16, respectively. This model
correctly predicts that galaxies follow a curved 2D surface in the 3D space of
log j⭑, log M⭑, and
β⭑. We find no statistically significant indication
that galaxies with classical and pseudo bulges follow different relations in this
space, although some differences are permitted within the observed scatter and the
inherent uncertainties in decomposing galaxies into disks and bulges. As a byproduct
of this analysis, we show that the
j⭑–M⭑ scaling relations for
disk-dominated galaxies from several previous studies are in excellent agreement
with each other. In addition, we resolve some conflicting claims about the
β⭑ dependence of the
j⭑–M⭑ scaling relations. The
results presented here reinforce and extend our earlier suggestion that the
j⭑–M⭑ diagram constitutes an
objective, physically motivated alternative to subjective classification schemes
such as the Hubble sequence.
The Dragonfly Nearby Galaxies Survey. V. HST/ACS observations of 23 low surface brightness objects in the fields of NGC 1052, NGC 1084, M96, and NGC 4258
Y. Cohen, P. van Dokkum, S. Danieli, A.J. Romanowsky, R. Abraham, A. Merritt, J. Zhang, L. Mowla, J.M.D. Kruijssen, C. Conroy, A. Wasserman
The Astrophysical Journal, Volume 868, Number 2, 2018 December 1, Article 96 (14pp)
ADS |
ApJ |
arXiv:1807.06016
We present Hubble Space Telescope/Advanced Camera for Surveys (ACS) imaging
of 23 very low surface brightness (μe,V ~ 25–27.5)
galaxies detected in the fields of four nearby galaxy groups. These objects were
selected from deep optical imaging obtained with the Dragonfly Telephoto Array.
Seven are newly identified, while most of the others had been seen previously in
visual surveys of deep photographic plates and more recent surveys. Few have
previously been studied in detail. From the ACS images, we measure distances to the
galaxies using both the tip of the red giant branch method and the surface
brightness fluctuations method. We demonstrate that the two methods are consistent
with each other in the regime where both can be applied. The distances to 15 out of
20 galaxies with stable measurements are consistent with that of the targeted group
within errors. This suggests that assuming group membership based solely on
projected proximity is ~75% successful in this regime. The galaxies are nearly
round, with a median axis ratio of 0.85, and visually resemble dwarf spheroidal
galaxies. The objects have a range of sizes, from Re = 0.4 kpc to
Re = 1.8 kpc, with a median <Re> = 1.0 kpc.
They range in luminosity from MV = –11.4 to
MV = –15.6, with a median <MV> =
–12.4. Galaxies with Re ~ 1 kpc and MV
~ –12 are fairly rare in the Local Group, but we find many of them in this
relatively small sample. Four of the objects fall in the class of ultra-diffuse
galaxies, with Re > 1.5 kpc and μ0,V > 24
mag arcsec–2, including the recently identified dark matter
deficient galaxy NGC 1052-DF2.
The extended Planetary Nebula Spectrograph (ePN.S) early-type
galaxy survey: The kinematic diversity of stellar halos and the
relation between halo transition scale and stellar mass
C. Pulsoni, O. Gerhard, M. Arnaboldi, L. Coccato, A. Longobardi, N.R. Napolitano, E. Moylan, C. Narayan, V. Gupta, A. Burkert, M. Capaccioli, A.L. Chies-Santos, A. Cortesi, K.C. Freeman, K. Kuijken, M.R. Merrifield, A.J. Romanowsky, C. Tortora
Astronomy & Astrophysics, Volume 618, October 2018, Article A94 (50pp)
ADS |
A&A |
arXiv:1712.05833
Press coverage:
ING
In the hierarchical two-phase formation scenario, the halos of early type
galaxies (ETGs) are expected to have different physical properties from the
galaxies' central regions. The ePN.S survey characterizes the kinematic
properties of ETG halos using planetary nebulae (PNe) as tracers, overcoming
the limitations of absorption line spectroscopy at low surface brightness.
We present two-dimensional velocity and velocity dispersion fields for 33 ETGs,
including fast (FRs) and slow rotators (SRs). The velocity fields were
reconstructed from the measured PN velocities using an adaptive kernel
procedure validated with simulations, and extend to a median of 5.6 effective
radii (Re). We complemented the PN kinematics with absorption
line data from the literature, for a complete description of the kinematics
from the center to the outskirts.
ETGs typically show a kinematic transition between inner regions and halo.
Estimated transition radii in units of Re anti-correlate with
stellar mass. SRs have increased but still modest rotational support at large
radii. Most of the FRs show a decrease in rotation, due to the fading of the
inner disk in the outer, more slowly rotating spheroid. 30% of the FRs are
dominated by rotation also at large radii. Most ETGs have flat or slightly
falling halo velocity dispersion profiles, but 15% of the sample have steeply
falling profiles. All of the SRs and 40% of the FRs show signatures of triaxial
halos such as kinematic twists or misalignments. We show with illustrative
photometric models that this is consistent with the distribution of isophote
twists from extended photometry.
ETGs have more diverse kinematic properties in their halos than in the central
regions. FRs do contain inner disk components but these frequently fade in
outer spheroids which are often triaxial. The observed kinematic transition to
the halo and its dependence on stellar mass is consistent with ΛCDM
simulations and supports a two-phase formation scenario.
The Fornax Cluster VLT Spectroscopic Survey – I. VIMOS spectroscopy of
compact stellar systems in the Fornax core region
V. Pota, N.R. Napolitano, M. Hilker, M. Spavone, C. Schulz, M. Cantiello, C. Tortora, E. Iodice,
M. Paolillo, R. D'Abrusco, M. Capaccioli, T. Puzia, R.F. Peletier, A.J. Romanowsky,
G. van den Ven, C. Spiniello, M. Norris, T. Lisker, R. Munoz, P. Schipani, P. Eigenthaler,
M.A. Taylor, R. Sánchez-Janssen, Y. Ordenes-Briceño
Monthly Notices of the Royal Astronomical Society, Volume 481, Issue 2, 2018 December 1, pp. 1744–1756
ADS |
MNRAS |
arXiv:1803.03275
We present the results of a wide spectroscopic survey aimed at detecting extragalactic
globular clusters (GCs) in the core of the Fornax cluster. About 4500 low-resolution
spectra (from 4800 to 10000 Å) were observed in 25 VLT/VIMOS masks covering the
central 1 deg2 around the dominant galaxy NGC 1399 corresponding to ~175
kpc galactocentric radius. We describe the methodology used for data reduction and
data analysis. We found a total of 387 unique physical objects (372 GCs and 15
ultracompact dwarfs) in the field covered by our observations. Most of these objects
lie in the region of NGC 1399 halo, with only 10 per cent likely belonging to other
giant galaxies 48 of these objects have a literature counterpart. The new VIMOS data
set is complementary to the many GC catalogues already present in the literature and
it brings the total number of tracer particles around NGC 1399 to more than 1130
objects. With this comprehensive radial velocity sample, we have found that the
velocity dispersion of the GC population (equally for red and blue GC populations)
shows a relatively sharp increase from low velocity dispersion
(~250–350 km s–1) to high velocity dispersion
(~300–400 km s–1) at projected radius of ≈10 arcmin
(~60 kpc) from the galaxy centre. This suggests that at a projected radius of
≈60 kpc both blue and red GC populations begin to be governed by the dominating Fornax cluster potential, rather than by the central NGC 1399 galaxy potential. This kinematic evidence corroborates similar results found using surface brightness analysis and planetary nebulae kinematics.
The distance of the dark matter deficient galaxy NGC1052-DF2
P. van Dokkum, S. Danieli, Y. Cohen, A.J. Romanowsky, C. Conroy
The Astrophysical Journal Letters, Volume 864, Number 1, 2018 September 1, Article L18 (7pp)
ADS |
ApJL |
arXiv:1807.06025
We recently inferred that the galaxy NGC 1052–DF2 has little or no dark matter and a rich
system of unusual globular clusters. We assumed that the galaxy is a satellite of the luminous
elliptical galaxy NGC 1052 at ≈20 Mpc, on the basis of its surface brightness fluctuations
(SBFs) distance of 19.0 ± 1.7 Mpc, its radial velocity of ≈1800 km s–1,
and its projected position. Here we analyze the color–magnitude diagram (CMD) of NGC
1052–DF2, following the suggestion by Trujillo et al. that the tip of the red giant branch
(TRGB) can be detected in currently available Hubble Space Telescope (HST) data and
the galaxy is at ~13 Mpc. Using fully populated galaxy models we show that the CMD is strongly
influenced by blends. These blends produce a "phantom" TRGB ~2 times brighter than the true TRGB,
which can lead to erroneous distance estimates ~1.4 times smaller than the actual distance. We
compare NGC 1052‐DF2 to model images as well as other galaxies in our HST sample, and show that
the large population of unblended RGB stars expected for distances of ~13 Mpc is not detected. We
also provide a new distance measurement to NGC 1052–DF2 that is free of calibration
uncertainties, by anchoring it to a satellite of the megamaser host galaxy NGC 4258. From a
megamaser-TRGB-SBF distance ladder we obtain D = 18.7 ± 1.7 Mpc, consistent with our
previous measurement and with the distance to the elliptical galaxy NGC 1052.
The SLUGGS Survey: the inner dark matter density slope of the massive elliptical galaxy NGC 1407
A. Wasserman, A.J. Romanowsky, J. Brodie, P. van Dokkum, C. Conroy, A. Villaume, D.A. Forbes, J. Strader, A. Alabi, S. Bellstedt
The Astrophysical Journal, Volume 863, Number 2, 2018 August 20, Article 130 (20pp)
ADS |
ApJ |
arXiv:1712.01229
We investigate the dark matter density profile of the massive elliptical galaxy,
NGC 1407, by constructing spherically symmetric Jeans models of its field star and
globular cluster systems. Two major challenges in such models are the degeneracy
between the stellar mass and the dark matter halo profiles, and the degeneracy
between the orbital anisotropy of the tracer population and the total mass causing
the observed motions. We address the first issue by using new measurements of the
mass-to-light ratio profile from stellar population constraints that include a
radially varying initial mass function. To mitigate the mass–anisotropy
degeneracy, we make use of multiple kinematic tracers, including two
subpopulations of globular clusters in addition to the galaxy's field stars. We
create a hierarchical Bayesian model that addresses several often-neglected
systematic uncertainties, such as the statistical weight given to various data
sets and the adopted distance. After sampling the posterior probability
distribution with a Markov chain Monte Carlo method, we find evidence for a
central cusp with a log slope of
γ = 1.0+0.2–0.4(stat)+0.3–0.5(sys), with the quantified systematic
uncertainty dominated by choice of anisotropy profile. This is lower than expected
for dark matter halos that have undergone adiabatic contraction, supporting
inferences from gravitational lensing that some process has suppressed the
steepening of halos in massive galaxies. We also confirm radially biased orbits
for the metal-rich globular clusters and tangentially biased orbits for the
metal-poor globular clusters, which remains a puzzling finding for an
accretion-dominated halo.
A deficit of dark matter from Jeans modeling of the ultra-diffuse galaxy NGC 1052-DF2
A. Wasserman, A.J. Romanowsky, J. Brodie, P. van Dokkum, C. Conroy, R. Abraham, Y. Cohen, S. Danieli
The Astrophysical Journal Letters, Volume 863, Number 2, 2018 August 20, Article L15 (6pp)
ADS |
ApJL |
arXiv:1807.07069
The discovery of the ultra-diffuse galaxy NGC 1052-DF2 and its peculiar
population of star clusters has raised new questions about the connections
between galaxies and dark matter (DM) halos at the extremes of galaxy formation.
In light of debates over the measured velocity dispersion of its star clusters
and the associated mass estimate, we constrain mass models of DF2 using its
observed kinematics with a range of priors on the halo mass. Models in which the
galaxy obeys a standard stellar-halo mass relation are in tension with the data
and also require a large central density core. Better fits are obtained when the
halo mass is left free, even after accounting for increased model complexity.
The dynamical mass-to-light ratio for our model with a weak prior on the halo
mass is 1.7+0.7–0.5
M☉/L☉,V, consistent with
the stellar population estimate for DF2. We use tidal analysis to find that the
low-mass models are consistent with the undisturbed isophotes of DF2. Finally,
we compare with Local Group dwarf galaxies and demonstrate that DF2 is an
outlier in both its spatial extent and its relative DM deficit.
Chromodynamical analysis of lenticular galaxies using globular clusters and planetary nebulae
E.J.B. Zanatta, A. Cortesi, A.L. Chies-Santos, D.A. Forbes, A.J. Romanowsky, A.B. Alabi, L. Coccato, C. Mendes de Oliveira, J.P. Brodie, M. Merrifield
Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 4, 2018 October 1, pp. 5124–5135
ADS |
MNRAS |
arXiv:1806.11242
Recovering the origins of lenticular galaxies can shed light on the understanding of
galaxy and compare them with the kinematics of planetary nebulae (PNe). The PNe and GC
data come from the Planetary Nebulae Spectrograph and the SLUGGS Surveys. Through
photometric spheroid-disc decomposition and PNe kinematics. we find the probability
for a given GC to belong to either the spheroid or the disc of its host galaxy or be
rejected from the model. We find that there is no correlation between the components
that the GCs are likely to belong to and their colours. Particularly, for NGC 2768, we
find that its red GCs display rotation preferentially at inner radii (Re < 1).
In the case of the GC system of NGC 3115, we find a group of GCs with similar
kinematics that are not likely to belong to either its spheroid or disc. For NGC 7457,
we find that 70 per cent of its GCs are likely to belong to the disc. Overall, our
results suggest that these galaxies assembled into S0s through different evolutionary
paths. Mergers seem to have been very important for NGC 2768 and NGC 3115 while
NGC 7457 is more likely to have experienced secular evolution.
A luminous X-ray outburst from an intermediate-mass black hole in an off-centre star cluster
D. Lin, J. Strader, E.R. Carrasco, D. Page, A.J. Romanowsky, J. Homan, J.A. Irwin, R.A. Remillard, O. Godet, N.A. Webb, H. Baumgardt, R. Wijnands, D. Barret, P.-A. Duc, J.P. Brodie, S.D.J. Gwyn
Nature Astronomy, Volume 2, 2018 August, pp. 656–661
ADS |
Nature Astronomy |
free access |
arXiv:1806.05692
Press coverage:
ESA |
ScienceDaily
A unique signature for the presence of massive black holes in very dense stellar regions is
occasional giant-amplitude outbursts of multi-wavelength radiation from tidal disruption and
subsequent accretion of stars that make a close approach to the black holes. Previous strong tidal
disruption event (TDE) candidates were all associated with the centres of largely isolated galaxies.
Here, we report the discovery of a luminous X-ray outburst from a massive star cluster at a
projected distance of 12.5 kpc from the centre of a large lenticular galaxy. The luminosity peaked
at ~1043 erg s–1 and decayed systematically over 10 years, approximately
following a trend that supports the identification of the event as a TDE. The X-ray spectra were all
very soft, with emission confined to be ≲3.0 keV, and could be described with a standard
thermal disk. The disk cooled significantly as the luminosity decreased—a key thermal-state
signature often observed in accreting stellar-mass black holes. This thermal-state signature,
coupled with very high luminosities, ultrasoft X-ray spectra and the characteristic power-law
evolution of the light curve, provides strong evidence that the source contains an intermediate-mass
black hole with a mass tens of thousand times that of the solar mass. This event demonstrates that
one of the most effective means of detecting intermediate-mass black holes is through X-ray flares
from TDEs in star clusters.
Origins of ultradiffuse galaxies in the Coma cluster – II. Constraints from their stellar populations
A. Ferré-Mateu, A. Alabi, D.A. Forbes, A.J. Romanowsky, J. Brodie,
V. Pandya, I. Martín-Navarro, S. Bellstedt, A. Wasserman, M.B. Stone, N. Okabe
Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 4, 2018 October 1, pp. 4891–4906
ADS |
MNRAS |
arXiv:1801.09695
In this second paper of the series we study, with new Keck/DEIMOS spectra, the stellar
populations of seven spectroscopically confirmed ultradiffuse galaxies (UDGs) in the Coma
cluster. We find intermediate to old ages (~7 Gyr), low metallicities ([Z/H] ~
–0.7 dex) and mostly supersolar abundance patterns ([Mg/Fe] ~ 0.13 dex). These
properties are similar to those of low-luminosity (dwarf) galaxies inhabiting the same
area in the cluster and are mostly consistent with being the continuity of the stellar
mass scaling relations of more massive galaxies. These UDGs' star formation histories
imply a relatively recent infall into the Coma cluster, consistent with the theoretical
predictions for a dwarf-like origin. However, considering the scatter in the resulting
properties and including other UDGs in Coma, together with the results from the velocity
phase-space study of the Paper I in this series, a mixed-bag of origins is needed to
explain the nature of all UDGs. Our results thus reinforce a scenario in which many UDGs
are field dwarfs that become quenched through their later infall onto cluster
environments, whereas some UDGs could be genuine primordial galaxies that failed to
develop due to an early quenching phase. The unknown proportion of dwarf-like to
primordial-like UDGs leaves the enigma of the nature of UDGs still open.
Origins of ultradiffuse galaxies in the Coma cluster – I. Constraints from velocity phase space
A. Alabi, A. Ferré-Mateu, A.J. Romanowsky, J. Brodie, D.A. Forbes, A. Wasserman,
S. Bellstedt, I. Martín-Navarro, V. Pandya, M.B. Stone, N. Okabe
Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 3, 2018 September 21, pp. 3308–3318
ADS |
MNRAS |
arXiv:1801.09686
We use Keck/DEIMOS spectroscopy to confirm the cluster membership of 16
ultradiffuse galaxies (UDGs) in the Coma cluster, bringing the total number of
spectroscopically confirmed UDGs from the Yagi et al. (Y16) catalogue to 25.
We also identify a new cluster background UDG, confirming that most (~95 per
cent) of the UDGs in the Y16 catalogue belong to the Coma cluster. In this
pilot study of Coma UDGs in velocity phase space, we find evidence of a
diverse origin for Coma cluster UDGs, similar to normal dwarf galaxies. Some
UDGs in our sample are consistent with being late infalls into the cluster
environment, while some may have been in the cluster for ≥ 8 Gyr. The late
infallen UDGs have higher absolute relative line-of-sight velocities, bluer
optical colours, and within the projected cluster core, are smaller in size,
compared to the early infalls. The early infall UDGs, which may also have
formed in situ, have been in the cluster environment for as long as the
most luminous galaxies in the Coma cluster, and they may be failed galaxies
that experienced star formation quenching at earlier epochs.
The contribution of HI-bearing ultra-diffuse galaxies to the cosmic number density of galaxies
M.G. Jones, E. Papastergis, V. Pandya, L. Leisman, A.J. Romanowsky, L.Y.A. Yung, R.S. Somerville, E.A.K. Adams
Astronomy & Astrophysics, Volume 614, June 2018, Article A21 (11pp)
ADS |
A&A |
arXiv:1712.01855
We estimate the cosmic number density of the recently identified class of
HI-bearing ultra-diffuse sources (HUDs) based on the completeness
limits of the ALFALFA survey. These objects have HI masses approximately in the
range 8:5 < log MHI / M☉ < 9.5, average
r-band surface brightnesses fainter than 24 mag arcsec–2,
half-light radii greater than 1.5 kpc, and are separated from neighbours by
at least 350 kpc. In this work we demonstrate that they contribute at most 6% of
the population of HI-bearing dwarfs detected by ALFALFA (with similar HI masses),
have a total cosmic number density of (1.5 ± 0.6) × 10–3
Mpc–3, and an HI mass density of (6.0 ± 0.8) ± 105
M☉ Mpc–3. We estimate that this is similar
to the total cosmic number density of ultra-diffuse galaxies (UDGs) in groups and
clusters, and conclude that the relation between the number of UDGs hosted in a halo
and the halo mass must have a break below M200 ~ 1012
M☉ in order to account for the abundance of HUDs in the field.
The distribution of the velocity widths of HUDs rises steeply towards low values,
indicating a preference for slow rotation rates compared to the global HI-rich dwarf
population. These objects were already included in previous measurements of the HI mass
function, but have been absent from measurements of the galaxy stellar mass function
owing to their low surface brightness. However, we estimate that due to their low
number density the inclusion of HUDs would constitute a correction of less than 1%.
Comparison with the Santa Cruz semi-analytic model shows that it produces HI-rich
central UDGs that have similar colours to HUDs, but that these UDGs are currently
produced in a much greater number. While previous results from this sample have
favoured formation scenarios where HUDs form in high spin-parameter halos, comparisons
with recent results which invoke that formation mechanism reveal that this model
produces an order of magnitude more field UDGs than we observe in the HUD population,
and these have an occurrence rate (relative to other dwarfs) that is approximately
double what we observe. In addition, the colours of HUDs are bluer than predicted,
although we suspect this is due to a systematic problem in reproducing the star
formation histories of low-mass galaxies rather than being specific to the
ultra-diffuse nature of these sources.
A 3.5-million Solar masses black hole in the centre of the ultracompact dwarf galaxy fornax UCD3
A.V. Afanasiev, I.V. Chilingarian, S. Mieske, K.T. Voggel, A. Picotti, M. Hilker, A. Seth,
N. Neumayer, M. Frank, A.J. Romanowsky, G. Hau, H. Baumgardt, C. Ahn, J. Strader,
M. den Brok, R. McDermid, L. Spitler, J. Brodie, J.L. Walsh
Monthly Notices of the Royal Astronomical Society, Volume 477, Issue 4, 2018 July 11, pp. 4856–4865
ADS |
MNRAS |
arXiv:1804.02938
The origin of ultracompact dwarfs (UCDs), a class of compact stellar systems discovered
two decades ago, still remains a matter of debate. Recent discoveries of central
supermassive black holes in UCDs likely inherited from their massive progenitor galaxies
provide support for the tidal stripping hypothesis. At the same time, on statistical
grounds, some massive UCDs might be representatives of the high luminosity tail of the
globular cluster luminosity function. Here we present a detection of a
3.3+1.4–1.2 106 M☉ black
hole (1σ uncertainty) in the centre of the UCD3 galaxy in the Fornax
cluster, which corresponds to 4 per cent of its stellar mass. We performed isotropic
Jeans dynamical modelling of UCD3 using internal kinematics derived from adaptive
optics-assisted observations with the SINFONI spectrograph and seeing limited data
collected with the FLAMES spectrograph at the ESO VLT. We rule out the zero black hole
mass at the 3σ confidence level when adopting a mass-to-light ratio
inferred from stellar populations. This is the fourth supermassive black hole found in a
UCD and the first one in the Fornax cluster. Similarly to other known UCDs that harbour
black holes, UCD3 hosts metal rich stars enhanced in α-elements that
support the tidal stripping of a massive progenitor as its likely formation scenario.
We estimate that up to 80 per cent of luminous UCDs in galaxy clusters host central
black holes. This fraction should be lower for UCDs in groups, because their progenitors
are more likely to be dwarf galaxies, which do not usually host black holes massive
enough to be detected.
The black hole in the most massive ultracompact dwarf galaxy M59-UCD3
C.P. Ahn, A.C. Seth, M. Cappellari, D. Krajnović, J. Strader, K.T. Voggel, J.L. Walsh,
A. Bahramian, H. Baumgardt, J. Brodie, I. Chilingarian, L. Chomiuk, M. den Brok, M. Frank,
M. Hilker, R.M. McDermid, S. Mieske, N. Neumayer, D.D. Nguyen, R. Pechetti, A.J. Romanowsky,
L. Spitler
The Astrophysical Journal, Volume 858, Number 2, 2018 May 11, Article 102 (16pp)
ADS |
ApJ |
arXiv:1804.02399
We examine the internal properties of the most massive ultracompact dwarf galaxy (UCD),
M59-UCD3, by combining adaptive-optics-assisted near-IR integral field spectroscopy from
Gemini/NIFS and Hubble Space Telescope (HST) imaging. We use the multiband
HST imaging to create a mass model that suggests and accounts for the presence of
multiple stellar populations and structural components. We combine these mass models
with kinematics measurements from Gemini/NIFS to find a best-fit stellar mass-to-light
ratio (M/L) and black hole (BH) mass using Jeans anisotropic models (JAMs),
axisymmetric Schwarzschild models, and triaxial Schwarzschild models. The best-fit
parameters in the JAM and axisymmetric Schwarzschild models have BHs between 2.5 and 5.9
million solar masses. The triaxial Schwarzschild models point toward a similar BH mass
but show a minimum χ2 at a BH mass of ~0. Models with a BH in all
three techniques provide better fits to the central Vrms profiles, and
thus we estimate the BH mass to be 4.2–1.7+2.1 ×
106 M☉ (estimated 1σ uncertainties). We
also present deep radio imaging of M59-UCD3 and two other UCDs in Virgo with dynamical
BH mass measurements, and we compare these to X-ray measurements to check for
consistency with the fundamental plane of BH accretion. We detect faint radio emission
in M59cO but find only upper limits for M60-UCD1 and M59-UCD3 despite X-ray detections
in both these sources. The BH mass and nuclear light profile of M59-UCD3 suggest that it is the tidally stripped remnant of a ~109–1010
M☉ galaxy.
The stellar populations of two ultra-diffuse galaxies from optical and near-infrared photometry
V. Pandya, A.J. Romanowsky, S. Laine, J.P. Brodie, B.D. Johnson, W. Glaccum,
A. Villaume, J.-C. Cuillandre, S. Gwyn, J. Krick, R. Lasker, I. Martín-Navarro,
D. Martinez-Delgado, P. van Dokkum
The Astrophysical Journal, Volume 858, Number 1, 2018 May 1, Article 29 (23pp)
ADS |
ApJ |
arXiv:1711.05272
We present observational constraints on the stellar populations of two ultra-diffuse
galaxies (UDGs) using optical through near-infrared (NIR) spectral energy distribution (SED)
fitting. Our analysis is enabled by new Spitzer-IRAC 3.6 and 4.5 μm
imaging, archival optical imaging, and the prospector fully Bayesian SED fitting framework.
Our sample contains one field UDG (DGSAT I), one Virgo cluster UDG (VCC 1287), and one Virgo
cluster dwarf elliptical for comparison (VCC 1122). We find that the optical–NIR colors
of the three galaxies are significantly different from each other. We infer that VCC 1287
has an old (≳7.7 Gyr) and surprisingly metal-poor
([Z/Z☉] ≲ –1.0) stellar population, even after
marginalizing over uncertainties on diffuse interstellar dust. In contrast, the field UDG
DGSAT I shows evidence of being younger than the Virgo UDG, with an extended star formation
history and an age posterior extending down to ~3 Gyr. The stellar metallicity of DGSAT I is
sub-solar but higher than that of the Virgo UDG, with
[Z/Z☉] = –0.63–0.62+0.35
in the case of exactly zero diffuse interstellar dust, DGSAT I may even have solar
metallicity. With VCC 1287 and several Coma UDGs, a general picture is emerging where
cluster UDGs may be "failed" galaxies, but the field UDG DGSAT I seems more consistent with
a stellar feedback-induced expansion scenario. In the future, our approach can be applied to
a large and diverse sample of UDGs down to faint surface brightness limits, with the goal of
constraining their stellar ages, stellar metallicities, and circumstellar and diffuse
interstellar dust content.
Upper limits on the presence of central massive black holes in two ultra-compact dwarf galaxies in Centaurus A
K.T. Voggel, A.C. Seth, N. Neumayer, S. Mieske, I. Chilingarian, C. Ahn, H. Baumgardt,
M. Hilker, D.D. Nguyen, A.J. Romanowsky, J.L. Walsh, M. den Brok, J. Strader
The Astrophysical Journal, Volume 858, Number 1, 2018 May 1, Article 20 (15pp)
ADS |
ApJ |
arXiv:1803.09750
The recent discovery of massive black holes (BHs) in the centers of high-mass ultra-compact
dwarf galaxies (UCDs) suggests that at least some are the stripped nuclear star clusters of
dwarf galaxies. We present the first study that investigates whether such massive BHs, and
therefore stripped nuclei, also exist in low-mass (M < 107
M☉) UCDs. We constrain the BH masses of two UCDs located in Centaurus A (UCD 320 and UCD 330) using Jeans modeling of the resolved stellar kinematics from adaptive
optics data obtained with the SINFONI integral field spectrograph at the Very Large Telescope
(VLT/SINFONI). No massive BHs are found in either UCD. We find a 3σ upper limit on
the central BH mass in UCD 330 of M● < 1.0 × 105
M☉, which corresponds to 1.7% of the total mass. This excludes a
high-mass fraction BH and would only allow low-mass BHs similar to those claimed to be
detected in Local Group globular clusters. For UCD 320, poorer data quality results in a less
constraining 3σ upper limit of M● <
1.0 × 106 M☉, which is equal to 37.7% of the total
mass. The dynamical mass-to-light ratios of UCD 320 and UCD 330 are not inflated compared to
predictions from stellar population models. The non-detection of BHs in these low-mass UCDs is
consistent with the idea that elevated dynamical mass-to-light ratios do indicate the presence
of a substantial BH. Although no massive BHs are detected, these systems could still be
stripped nuclei. The strong rotation (v/σ of 0.3–0.4) in both UCDs and
the two-component light profile in UCD 330 support the idea that these UCDs may be stripped
nuclei of low-mass galaxies whose BH occupation fraction is not yet known.
The SLUGGS survey: a comparison of total-mass profiles of early-type galaxies from observations and cosmological simulations, to ~4 effective radii
S. Bellstedt, D.A. Forbes, A.J. Romanowsky, R.-S. Remus, A.R.H. Stevens, J.P. Brodie, A. Poci, R. McDermid, A. Alabi, L. Chevalier, C. Adams, A. Ferré-Mateu, A. Wasserman, V. Pandya
Monthly Notices of the Royal Astronomical Society, Volume 476, Issue 4, 2018 June 1, pp. 4543–4564
ADS |
MNRAS |
erratum |
arXiv:1803.02373
We apply the Jeans Anisotropic Multi-Gaussian Expansion dynamical modelling
method to SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey data of
early-type galaxies in the stellar mass range 1010 <
M*/M☉ < 1011.6 that cover a large
radial range of 0.1–4.0 effective radii. We combine SLUGGS and
ATLAS3D data sets to model the total-mass profiles of a sample of 21
fast-rotator galaxies, utilizing a hyperparameter method to combine the two
independent data sets. The total-mass density profile slope values derived for
these galaxies are consistent with those measured in the inner regions of
galaxies by other studies. Furthermore, the total-mass density slopes
(γtot) appear to be universal over this broad stellar
mass range, with an average value of γtot = –2.24
± 0.05, i.e. slightly steeper than isothermal. We compare our results to model galaxies from the Magneticum and EAGLE cosmological hydrodynamic
simulations, in order to probe the mechanisms that are responsible for varying
total-mass density profile slopes. The simulated-galaxy slopes are shallower than
the observed values by ~ 0.3–0.5, indicating that the physical processes
shaping the mass distributions of galaxies in cosmological simulations are still
incomplete. For galaxies with M* > 1010.7
M☉ in the Magneticum simulations, we identify a
significant anticorrelation between total-mass density profile slopes and the
fraction of stellar mass formed ex situ (i.e. accreted), whereas this
anticorrelation is weaker for lower stellar masses, implying that the measured
total-mass density slopes for low-mass galaxies are less likely to be determined
by merger activity.
An enigmatic population of luminous globular clusters in a galaxy lacking dark matter
P. van Dokkum, Y. Cohen, S. Danieli, J.M.D. Kruijssen, A.J. Romanowsky, A. Merritt, R. Abraham, J. Brodie, C. Conroy, D. Lokhorst, L. Mowla, E. O'Sullivan, J. Zhang
The Astrophysical Journal Letters, Volume 856, Number 2, 2018 April 1, Article L30 (7pp)
ADS |
ApJL |
arXiv:1803.10240
Press coverage:
AAS Nova
We recently found an ultra diffuse galaxy (UDG) with a half-light radius of
Re = 2.2 kpc and little or no dark matter. The total mass of
NGC1052-DF2 was measured from the radial velocities of bright compact objects
that are associated with the galaxy. Here, we analyze these objects using a
combination of Hubble Space Telescope (HST) imaging and Keck
spectroscopy. Their average size is <rh> = 6.2 ± 0.5 pc and
their average ellipticity is <ε > = 0.18 ± 0.02. From a
stacked Keck spectrum we derive an age of ≳ 9 Gyr and a metallicity of
[Fe/H] = –1.35 ± 0.12. Their properties are similar to ω
Centauri, the brightest and largest globular cluster in the Milky Way, and our
results demonstrate that the luminosity function of metal-poor globular clusters
is not universal. The fraction of the total stellar mass that is in the globular
cluster system is similar to that in other UDGs, and consistent with "failed
galaxy" scenarios, where star formation terminated shortly after the clusters
were formed. However, the galaxy is a factor of ~1000 removed from the relation
between globular cluster mass and total galaxy mass that has been found for other
galaxies, including other UDGs. We infer that a dark matter halo is not a
prerequisite for the formation of metal-poor globular cluster-like objects in
high-redshift galaxies.
A galaxy lacking dark matter
P. van Dokkum, S. Danieli, Y. Cohen, A. Merritt, A.J. Romanowsky, R. Abraham, J. Brodie, C. Conroy, D. Lokhorst, L. Mowla, E. O'Sullivan, J. Zhang
Nature, Volume 555, Number 7698, 2018 March 29, pp. 629–632
ADS |
Nature |
arXiv:1803.10237
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HST #1 |
HST #2 |
HST #3 |
Keck |
Gemini |
Yale |
SJSU |
BBC |
Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that
the mass of the dark matter halo and the total stellar mass are coupled through a function
that varies smoothly with mass. Their average ratio
Mhalo/Mstars has a minimum of about 30 for galaxies
with stellar masses near that of the Milky Way (approximately 5×1010
solar masses) and increases both towards lower masses and towards higher masses. The
scatter in this relation is not well known; it is generally thought to be less than a
factor of two for massive galaxies but much larger for dwarf galaxies. Here we report
the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse
galaxy NGC1052-DF2, which has a stellar mass of approximately 2×108
solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per
second with 90 per cent confidence, and we determine from this that its total mass
within a radius of 7.6 kiloparsecs is less than 3.4×108 solar masses.
This implies that the ratio Mhalo/Mstars is of
order unity (and consistent with zero), a factor of at least 400 lower than expected.
NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter
on galactic scales.
Black-hole-regulated star formation in massive galaxies
I. Martín-Navarro, J.P. Brodie, A.J. Romanowsky, T. Ruiz-Lara, G. van de Ven
Nature, Volume 553, Number 7688, 2018 January 18, pp. 307–309
ADS |
Nature |
arXiv:1801.00807
Press coverage:
UCSC |
SJSU |
Atlantic |
Newsweek |
The Register |
Gizmodo |
Universe Today
Supermassive black holes, with masses more than a million times that of the Sun, seem
to inhabit the centres of all massive galaxies. Cosmologically motivated theories of
galaxy formation require feedback from these supermassive black holes to regulate star
formation. In the absence of such feedback, state-of-the-art numerical simulations
fail to reproduce the number density and properties of massive galaxies in the local
Universe. There is, however, no observational evidence of this strongly coupled
coevolution between supermassive black holes and star formation, impeding our
understanding of baryonic processes within galaxies. Here we report that the star
formation histories of nearby massive galaxies, as measured from their integrated
optical spectra, depend on the mass of the central supermassive black hole. Our
results indicate that the black-hole mass scales with the gas cooling rate in the
early Universe. The subsequent quenching of star formation takes place earlier and
more efficiently in galaxies that host higher-mass central black holes. The observed
relation between black-hole mass and star formation efficiency applies to all
generations of stars formed throughout the life of a galaxy, revealing a continuous
interplay between black-hole activity and baryon cooling.
Deep Subaru Hyper Suprime-Cam observations of Milky Way satellites Columba I and Triangulum II
J.L. Carlin, D.J. Sand, R.R. Muñoz, K. Spekkens, B. Willman, D. Crnojević,
D.A. Forbes, J. Hargis, E. Kirby, A.H.G. Peter, A.J. Romanowsky, J. Strader
The Astronomical Journal, Volume 154, Number 6, 2017 December, Article 267 (10pp)
ADS |
AJ |
arXiv:1710.06444
We present deep, wide-field Subaru Hyper Suprime-Cam photometry of two recently
discovered satellites of the Milky Way (MW): Columba I (Col I) and Triangulum II
(Tri II). The color–magnitude diagrams of both objects point to exclusively old
and metal-poor stellar populations. We re-derive structural parameters and luminosities
of these satellites, and find MV, Col I = –4.2 ± 0.2 for Col I
and MV, Tri II = –1.2 ± 0.4 for Tri II, with corresponding
half-light radii of rh, Col I = 117 ± 17 pc and
rh, Tri II = 21 ± 4 pc. The properties of both systems are
consistent with observed scaling relations for MW dwarf galaxies. Based on archival
data, we derive upper limits on the neutral gas content of these dwarfs, and find that
they lack HI, as do the majority of observed satellites within the MW virial radius.
Neither satellite shows evidence of tidal stripping in the form of extensions or
distortions in matched-filter stellar density maps or surface-density profiles.
However, the smaller Tri II system is relatively metal-rich for its luminosity
(compared to other MW satellites), possibly because it has been tidally stripped.
Through a suite of orbit simulations, we show that Tri II is approaching pericenter of
its eccentric orbit, a stage at which tidal debris is unlikely to be seen. In addition,
we find that Tri II may be on its first infall into the MW, which helps explain its
unique properties among MW dwarfs. Further evidence that Tri II is likely an
ultra-faint dwarf comes from its stellar mass function, which is similar to those
of other MW dwarfs.
Initial mass function variability (or not) among low-velocity dispersion, compact stellar systems
A. Villaume, J. Brodie, C. Conroy, A.J. Romanowsky, P. van Dokkum
The Astrophysical Journal Letters, Volume 850, Number 1, 2017 November 20, Article L14 (6pp)
ADS |
ApJL |
arXiv:1710.11144
Analyses of strong gravitational lenses, galaxy-scale kinematics, and absorption-line
stellar population synthesis (SPS) have all concluded that the stellar initial mass
function (IMF) varies within the massive early-type galaxy (ETG) population. However, the
physical mechanism that drives variation in the IMF is an outstanding question. Here we use
new SPS models to consider a diverse set of compact, low-velocity dispersion stellar
systems: globular clusters (GCs), an ultra-compact dwarf (UCD), and the compact elliptical
(cE) galaxy M32. We compare our results to massive ETGs and available dynamical
measurements. We find that the GCs have stellar mass-to-light ratios (M/L)
that are either consistent with a Kroupa IMF or are slightly bottom-light, while the UCD
and cE have mildly elevated M/L. The separation in derived IMFs for systems
with similar metallicities and abundance patterns indicates that our SPS models can
distinguish abundance and IMF effects. Variation among the sample in this paper is only
~50% in normalized M/L compared to the ~4× among the ETG sample. This
suggests that metallicity is not the sole driver of IMF variability and additional
parameters need to be considered.
On the formation mechanisms of compact elliptical galaxies
A. Ferré-Mateu, D.A. Forbes, A.J. Romanowsky, J. Janz, C. Dixon
Monthly Notices of the Royal Astronomical Society, Volume 473, Issue 2, 2018 January 11, pp. 1819–1840
ADS |
MNRAS |
arXiv:1709.07012
In order to investigate the formation mechanisms of the rare compact elliptical (cE) galaxies, we have
compiled a sample of 25 cEs with good SDSS spectra, covering a range of stellar masses, sizes and
environments. They have been visually classified according to the interaction with their host,
representing different evolutionary stages. We have included clearly disrupted galaxies, galaxies that
despite not showing signs of interaction are located close to a massive neighbour (thus are good
candidates for a stripping process), and cEs with no host nearby. For the latter, tidal stripping is
less likely to have happened and instead they could simply represent the very low-mass, faint end of
the ellipticals. We study a set of properties (structural parameters, stellar populations, star
formation histories and mass ratios) that can be used to discriminate between an intrinsic or stripped
origin. We find that one diagnostic tool alone is inconclusive for the majority of objects. However,
if we combine all the tools a clear picture emerges. The most plausible origin, as well as the
evolutionary stage and progenitor type, can be then determined. Our results favour the stripping
mechanism for those galaxies in groups and clusters that have a plausible host nearby, but favours an
intrinsic origin for those rare cEs without a plausible host and that are located in looser
environments.
Extensive globular cluster systems associated with ultra diffuse galaxies in the Coma cluster
P. van Dokkum, R. Abraham, A.J. Romanowsky, J. Brodie, C. Conroy, S. Danieli, D. Lokhorst, A. Merritt, L. Mowla, J. Zhang
The Astrophysical Journal Letters, Volume 844, Number 1, 2017 July 20, Article L11 (7pp)
ADS |
ApJL |
arXiv:1705.08513
Press coverage:
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We present Hubble Space Telescope (HST) imaging of two ultra diffuse galaxies
(UDGs) with measured stellar velocity dispersions in the Coma cluster. The galaxies,
Dragonfly 44 and DFX1, have effective radii of 4.7 kpc and 3.5 kpc and velocity
dispersions of 47+8–6 km s–1 and
30+7–7 km s–1, respectively. Both
galaxies are associated with a striking number of compact objects, tentatively
identified as globular clusters: Ngc = 74 ± 18 for Dragonfly 44
and Ngc = 62 ± 17 for DFX1. The number of globular clusters is
much higher than expected from the luminosities of the galaxies but is consistent
with expectations from the empirical relation between dynamical mass and globular
cluster count defined by other galaxies. Combining our data with previous HST
observations of Coma UDGs we find that UDGs have a factor of
6.9+1.0-2.4 more globular clusters than other galaxies of the
same luminosity, in contrast to a recent study of a similar sample by Amorisco et
al., but consistent with earlier results for individual galaxies. The Harris et al.
relation between globular cluster count and dark matter
halo mass implies a median halo mass of Mhalo ~ 1.5 ×
1011 M☉ for the sixteen Coma UDGs that have been
observed with HST so far, with the largest and brightest having
Mhalo ~ 5 × 1011 M☉.
Constraining the physical state of the hot gas halos in NGC 4649 and NGC 5846
A. Paggi, D.-W. Kim, C. Anderson, D. Burke, R. D'Abrusco, G. Fabbiano, A. Fruscione,
T. Gokas, J. Lauer, M. McCollough, D. Morgan, A. Mossman, E. O'Sullivan,
G. Trinchieri, S. Vrtilek, S. Pellegrini, A.J. Romanowsky, J. Brodie
The Astrophysical Journal, Volume 844, Number 1, 2017 July 20, Article 5 (30pp)
ADS |
ApJ |
arXiv:1706.02303
We present results of a joint Chandra/XMM-Newton analysis of the early-type galaxies
NGC 4649 and NGC 5846 aimed at investigating differences between mass profiles derived
from X-ray data and those from optical data, to probe the state of the hot interstellar
medium (ISM) in these galaxies. If the hot ISM is at a given radius in hydrostatic
equilibrium (HE), the X-ray data can be used to measure the total enclosed mass of the
galaxy. Differences from optically derived mass distributions therefore yield
information about departures from HE in the hot halos. The X-ray mass profiles in
different angular sectors of NGC 4649 are generally smooth with no significant
azimuthal asymmetries within 12 kpc. Extrapolation of these profiles beyond this scale
yields results consistent with the optical estimate. However, in the central region
(r < 3 kpc) the X-ray data underpredict the enclosed mass, when compared with
the optical mass profiles. Consistent with previous results, we estimate a nonthermal
pressure component accounting for 30% of the gas pressure, likely linked to nuclear
activity. In NGC 5846 the X-ray mass profiles show significant azimuthal asymmetries,
especially in the NE direction. Comparison with optical mass profiles in this direction
suggests significant departures from HE, consistent with bulk gas compression and
decompression due to sloshing on ~15 kpc scales; this effect disappears in the NW
direction, where the emission is smooth and extended. In this sector we find consistent
X-ray and optical mass profiles, suggesting that the hot halo is not responding to
strong nongravitational forces.
The SLUGGS Survey: trails of SLUGGS galaxies in a modified spin-ellipticity diagram
S. Bellstedt, A.W. Graham, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 470, Issue 2, 2017 September 11, pp. 1321–1328
ADS |
MNRAS |
arXiv:1706.01469
We present radial tracks for four early-type galaxies with embedded
intermediate-scale discs in a modified spin-ellipticity diagram. Here, each
galaxy's spin and ellipticity profiles are shown as a radial track, as opposed to
a single, flux-weighted aperture-dependent value as is common in the literature.
The use of a single ellipticity and spin parameter is inadequate to capture the
basic nature of these galaxies, which transition from fast to slow rotation as one
moves to larger radii where the disc ceases to dominate. After peaking, the four
galaxy's radial tracks feature a downturn in both ellipticity and spin with
increasing radius, differentiating them from elliptical galaxies, and from
lenticular galaxies whose discs dominate at large radii. These galaxies are
examples of so-called discy elliptical galaxies, which are a morphological hybrid
between elliptical (E) and lenticular (S0) galaxies and have been designated ES
galaxies. The use of spin-ellipticity tracks provides extra structural information
about individual galaxies over a single aperture measure. Such tracks provide a
key diagnostic for classifying early-type galaxies, particularly in the era of 2D
kinematic (and photometric) data beyond one effective radius.
The stellar initial mass function in early-type galaxies from absorption line spectroscopy. III. Radial gradients
P. van Dokkum, C. Conroy, A. Villaume, J. Brodie, A.J. Romanowsky
The Astrophysical Journal, Volume 841, Number 2, 2017 June 1, Article 68 (23pp)
ADS |
ApJ |
arXiv:1611.09859
There is good evidence that the centers of massive early-type galaxies have a bottom-heavy
stellar initial mass function (IMF) compared to that of the Milky Way. Here we study the
radial variation of the IMF within such galaxies, using a combination of high-quality Keck
spectroscopy and a new suite of stellar population synthesis models that cover a wide
range in metallicity. As in the previous studies in this series, the models are fitted
directly to the spectra and treat all elemental abundance ratios as free parameters. Using
newly obtained spectroscopy for six galaxies, including deep data extending to
~1 Re for the galaxies NGC 1407, NGC 1600, and NGC 2695, we find that
the IMF varies strongly with galactocentric radius. For all six galaxies the IMF is
bottom-heavy in the central regions, with average mass-to-light ratio "mismatch" parameter
α ≡ (M/L)/(M/L)MW ≈ 2.5
at R = 0. The IMF rapidly becomes more bottom-light with increasing radius,
flattening off near the Milky Way value (α ≈ 1.1) at R > 0.4
Re. A consequence is that the luminosity-weighted average IMF depends on
the measurement aperture: within R = Re we find
<α>L = 1.3–1.5, consistent with recent lensing and
dynamical results from SLACS and ATLAS3D. Our results are also consistent with
several earlier studies that were based on analyses of radial gradients of line indices.
The observed IMF gradients support galaxy formation models in which the central regions of
massive galaxies had a different formation history than their outer parts. Finally, we
make use of the high signal-to-noise central spectra of NGC 1407 and NGC 2695 to
demonstrate how we can disentangle IMF effects and abundance effects.
The HI content of isolated ultra-diffuse galaxies: A sign of multiple formation mechanisms?
E. Papastergis, E.A.K. Adams, A.J. Romanowsky
Astronomy & Astrophysics Letters, Volume 601, May 2017, Article L10 (4pp)
ADS |
A&A |
arXiv:1703.05610
Press coverage:
A&A
We report on the results of radio observations in the 21 cm emission line of atomic
hydrogen (HI) of four relatively isolated ultra-diffuse galaxies (UDGs): DGSAT I,
R-127-1, M-161-1, and SECCO-dI-2. Our Effelsberg observations resulted in
non-detections for the first three UDGs, and a clear detection for the last. DGSAT I,
R-127-1, and M-161-1 are quiescent galaxies with gas fractions that are much lower than
those of typical field galaxies of the same stellar mass. On the other hand,
SECCO-dI-2 is a star forming gas-rich dwarf, similar to two other field UDGs that have
literature HI data: SECCO-dI-1 and UGC 2162. This group of three gas-rich UDGs have
stellar and gaseous properties that are compatible with a recently proposed theoretical
mechanism for the formation of UDGs, based on feedback-driven outflows. In contrast,
the physical characteristics of R-127-1 and M-161-1 are puzzling, given their isolated
nature. We interpret this dichotomy in the gaseous properties of field UDGs as a sign
of the existence of multiple mechanisms for their formation, with the formation of the
quiescent gas-poor UDGs remaining a mystery.
The SLUGGS Survey: dark matter fractions at large radii and assembly epochs of early-type galaxies from globular cluster kinematics
A.B. Alabi, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader, J. Janz, C. Usher, L.R. Spitler, S. Bellstedt, A. Ferré-Mateu
Monthly Notices of the Royal Astronomical Society, Volume 468, Issue 4, 2017 July 11, pp. 3949–3964
ADS |
MNRAS |
arXiv:1701.05904
We use globular cluster kinematics data, primarily from the SAGES Legacy Unifying Globulars
and GalaxieS (SLUGGS) survey, to measure the dark matter fraction (fDM)
and the average dark matter density (<ρDM>) within the inner 5
effective radii (Re) for 32 nearby early-type galaxies (ETGs) with
stellar mass log (M*/M☉) ranging from 10.1 to 11.8.
We compare our results with a simple galaxy model based on scaling relations as well as
with cosmological hydrodynamical simulations where the dark matter profile has been
modified through various physical processes. We find a high fDM
(≥ 0.6) within 5 Re in most of our sample, which we interpret as a
signature of a late mass assembly history that is largely devoid of gas-rich major mergers.
However, around log (M*/M☉) ~ 11, there is a wide range
of fDM which may be challenging to explain with any single cosmological
model. We find tentative evidence that lenticulars (S0s), unlike ellipticals, have mass
distributions that are similar to spiral galaxies, with decreasing fDM
within 5 Re as galaxy luminosity increases. However, we do not find any
difference between the <ρDM> of S0s and ellipticals in our sample,
despite the differences in their stellar populations. We have also used
<ρDM> to infer the epoch of halo assembly (z ~ 2–4).
By comparing the age of their central stars with the inferred epoch of halo formation, we
are able to gain more insight into their mass assembly histories. Our results suggest a
fundamental difference in the dominant late-phase mass assembly channel between lenticulars
and elliptical galaxies.
Detection of supermassive black holes in two Virgo ultracompact dwarf galaxies
C.P. Ahn, A.C. Seth, M. den Brok, J. Strader, H. Baumgardt, R. van den Bosch, I. Chilingarian, M. Frank, M. Hilker, R. McDermid, S. Mieske, A.J. Romanowsky, L. Spitler, J. Brodie, N. Neumayer, J.L. Walsh
The Astrophysical Journal, Volume 839, Number 2, 2017 April 20, Article 72 (15pp)
ADS |
ApJ |
arXiv:1703.09221
Press coverage:
Utah |
New Scientist
We present the detection of supermassive black holes (BHs) in two Virgo
ultracompact dwarf galaxies (UCDs), VUCD3 and M59cO. We use adaptive optics
assisted data from the Gemini/NIFS instrument to derive radial velocity dispersion
profiles for both objects. Mass models for the two UCDs are created using
multi-band Hubble Space Telescope imaging, including the modeling of mild
color gradients seen in both objects. We then find a best-fit stellar
mass-to-light ratio (M/L) and BH mass by combining the kinematic
data and the deprojected stellar mass profile using Jeans Anisotropic Models.
Assuming axisymmetric isotropic Jeans models, we detect BHs in both objects with
masses of 4.4+2.5–3.0 × 106
M☉ in VUCD3 and 5.8+2.5–2.8
× 106 M☉ in M59cO (3σ
uncertainties). The BH mass is degenerate with the anisotropy parameter,
βz; for the data to be consistent with no BH requires
βz = 0.4 and βz = 0.6 for VUCD3 and
M59cO, respectively. Comparing these values with nuclear star clusters shows that,
while it is possible that these UCDs are highly radially anisotropic, it seems
unlikely. These detections constitute the second and third UCDs known to host
supermassive BHs. They both have a high fraction of their total mass in their BH;
~13% for VUCD3 and ~18% for M59cO. They also have low best-fit stellar M/Ls,
supporting the proposed scenario that most massive UCDs host high-mass fraction
BHs. The properties of the BHs and UCDs are consistent with both objects being the
tidally stripped remnants of ~109 M☉ galaxies.
Ultra-diffuse and ultra-compact galaxies in the Frontier Fields cluster Abell 2744
S. Janssens, R. Abraham, J. Brodie, D. Forbes, A.J. Romanowsky, P. van Dokkum
The Astrophysical Journal Letters, Volume 839, Number 1, 2017 April 10, Article L17 (5pp)
ADS |
ApJL |
arXiv:1701.00011
Press coverage:
phys.org
We report the discovery of a large population of ultra-diffuse galaxies (UDGs) in the
massive galaxy cluster Abell 2744 (z = 0.308) as observed by the Hubble
Frontier Fields program. Since this cluster is ~5 times more massive than Coma, our
observations allow us to extend 0.7 dex beyond the high-mass end of the relationship
between UDG abundance and cluster mass reported by van der Burg et al. Using the same
selection criteria as van der Burg et al., A2744 hosts an estimated 1961 ± 577
UDGs, 10 times the number in Coma. As noted by Lee & Jang, A2744 contains numerous
unresolved compact objects, which those authors identified predominantly as globular
clusters. However, these objects have luminosities that are more consistent with
ultra-compact dwarf (UCD) galaxies. The abundances of both UCDs and UDGs scale with
cluster mass as a power law with a similar exponent, although UDGs and UCDs have very
different radial distributions within the cluster. The radial surface density
distribution of UCDs rises sharply toward the cluster center, while the surface
density distribution of the UDG population is essentially flat. Together, these
observations hint at a picture where some UCDs in A2744 may have once been associated
with infalling UDGs. As UDGs fall in and dissolve, they leave behind a residue of
unbound UCDs.
The SLUGGS survey: using extended stellar kinematics to disentangle the
formation histories of low-mass S0 galaxies
S. Bellstedt, D.A. Forbes, C. Foster, A.J. Romanowsky, J.P. Brodie,
N. Pastorello, A. Alabi, A. Villaume
Monthly Notices of the Royal Astronomical Society, Volume 467, Issue 4, 2017 June 1, pp. 4540–4557
ADS |
MNRAS |
arXiv:1702.05099
We utilize the DEIMOS instrument on the Keck telescope to measure the wide-field stellar
kinematics of early-type galaxies as part of the SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey. In this paper, we focus on some of the lowest stellar mass lenticular galaxies
within this survey, namely NGC 2549, NGC 4474, NGC 4459 and NGC 7457, performing detailed
kinematic analyses out to large radial distances of ~2–3 effective radii. For NGC 2549,
we present the first analysis of data taken with the SuperSKiMS (Stellar Kinematics from
Multiple Slits) technique. To better probe kinematic variations in the outskirts of the SLUGGS
galaxies, we have defined a local measure of stellar spin. We use this parameter and identify
a clear separation in the radial behaviour of stellar spin between lenticular and elliptical
galaxies. We compare the kinematic properties of our galaxies with those from various simulated
galaxies to extract plausible formation scenarios. By doing this for multiple simulations, we
assess the consistency of the theoretical results. Comparisons to binary merger simulations
show that low-mass lenticular galaxies generally resemble the spiral progenitors more than the
merger remnants themselves, which is an indication that these galaxies are not formed through
merger events. We find, however, that recent mergers cannot be ruled out for some lenticular
galaxies.
The SLUGGS Survey: a catalog of over 4000 globular cluster radial
velocities in 27 nearby early-type galaxies
D.A. Forbes, A. Alabi, J.P. Brodie, A.J. Romanowsky, J. Strader, C. Foster,
C. Usher, L. Spitler, S. Bellstedt, N. Pastorello, A. Villaume, A. Wasserman, V. Pota
The Astronomical Journal, Volume 153, Number 3, 2017 March, Article 114 (10pp)
ADS |
AJ |
arXiv:1701.04835
Here, we present positions and radial velocities for over 4000 globular clusters (GCs) in 27
nearby early-type galaxies from the SLUGGS survey. The SLUGGS survey is designed to be
representative of elliptical and lenticular galaxies in the stellar mass range 10 < log
M*/M☉ < 11.7. The data have been obtained over many years,
mostly using the very stable multi-object spectrograph DEIMOS on the Keck II 10 m telescope.
Radial velocities are measured using the calcium triplet lines, with a velocity accuracy of
±10–15 km s–1. We use phase space diagrams (i.e.,
velocity–position diagrams) to identify contaminants such as foreground stars and
background galaxies, and to show that the contribution of GCs from neighboring galaxies is
generally insignificant. Likely ultra-compact dwarfs are tabulated separately. We find that
the mean velocity of the GC system is close to that of the host galaxy systemic velocity,
indicating that the GC system is in overall dynamical equilibrium within the galaxy potential.
We also find that the GC system velocity dispersion scales with host galaxy stellar mass,
in a similar manner to the Faber–Jackson relation for the stellar velocity dispersion.
Publication of these GC radial velocity catalogs should enable further studies in many areas,
such as GC system substructure, kinematics, and host galaxy mass measurements.
The SLUGGS Survey: stellar masses and effective radii of early-type galaxies
from Spitzer Space Telescope 3.6 μm imaging
D.A. Forbes, L. Sinpetru, G. Savorgnan, A.J. Romanowsky, C. Usher, J. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 464, Issue 4, 2017 February 01, pp. 4611–4623
ADS |
MNRAS |
arXiv:1610.02037
Galaxy starlight at 3.6 μm is an excellent tracer of stellar mass. Here we use the latest 3.6 μm
imaging from the Spitzer Space Telescope to measure the total stellar mass and effective radii
in a homogeneous way for a sample of galaxies from the SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey. These galaxies are representative of nearby early-type galaxies in the stellar mass
range of 10 < log M*/M☉ < 11.7 and our methodology can be applied
to other samples of early-type galaxies. We model each galaxy in 2D and estimate its total asymptotic
magnitude from a 1D curve-of-growth. Magnitudes are converted into stellar masses using a 3.6 μm
mass-to-light ratio from the latest stellar population models of Röck et al., assuming a Kroupa
initial mass function. We apply a ratio based on each galaxy's mean mass-weighted stellar age within
one effective radius (the mass-to-light ratio is insensitive to galaxy metallicity for the generally
old stellar ages and high metallicities found in massive early-type galaxies). Our 3.6 μm stellar
masses agree well with masses derived from 2.2 μm data. From the 1D surface brightness profile,
we fit a single Sérsic law, excluding the very central regions. We measure the effective
radius, Sérsic n parameter and effective surface brightness for each galaxy. We find
that galaxy sizes derived from shallow optical imaging and the 2MASS survey tend to underestimate
the true size of the largest, most massive galaxies in our sample. We adopt the 3.6 μm stellar
masses and effective radii for the SLUGGS survey galaxies.
Stellar populations across the black hole mass–velocity dispersion relation
I. Martín-Navarro, J.P. Brodie, R.C.E. van den Bosch, A.J. Romanowsky, D.A. Forbes
The Astrophysical Journal Letters, Volume 832, Number 1, 2016 November 20, Article L11 (5pp)
ADS |
ApJL |
arXiv:1609.05899
Coevolution between supermassive black holes (BH) and their host galaxies is universally adopted
in models for galaxy formation. In the absence of feedback from active galactic nuclei (AGNs),
simulated massive galaxies keep forming stars in the local universe. From an observational point
of view, however, such coevolution remains unclear. We present a stellar population analysis of
galaxies with direct BH mass measurements and the BH mass–σ relation as a
working framework. We find that over-massive BH galaxies, i.e., galaxies lying above the
best-fitting BH mass–σ line, tend to be older and more
α-element-enhanced than under-massive BH galaxies. The scatter in the BH
mass–[α/Fe] plane is significantly lower than that in the standard BH
mass–σ relation. We interpret this trend as an imprint of AGN feedback on
the star formation histories of massive galaxies.
Ultraluminous X-ray bursts in two ultracompact companions to nearby elliptical galaxies
J.A. Irwin, W.P. Maksym, G.R. Sivakoff, A.J. Romanowsky, D. Lin, T. Speegle, I. Prado, D. Mildebrath, J. Strader, J. Liu, J.M. Miller
Nature, Volume 538, Number 7625, 2016 October 20, pp. 356–358
ADS |
Nature |
News & Views |
arXiv:1610.05781
Press coverage:
Chandra |
New Scientist
A flaring X-ray source was found near the galaxy NGC 4697
(ref. 1). Two brief flares were seen, separated by four years. During
each flare, the flux increased by a factor of 90 on a timescale of about
one minute. There is no associated optical source at the position
of the flares, but if the source was at the distance of NGC 4697,
then the luminosities of the flares were greater than 1039 erg per
second. Here we report the results of a search of archival X-ray
data for 70 nearby galaxies looking for similar flares. We found two
ultraluminous flaring sources in globular clusters or ultracompact
dwarf companions of parent elliptical galaxies. One source flared
once to a peak luminosity of 9×1040 erg per second; the other flared
five times to 1040 erg per second. The rise times of all of the flares
were less than one minute, and the flares then decayed over about an
hour. When not flaring, the sources appear to be normal accreting
neutron-star or black-hole X-ray binaries, but they are located in old
stellar populations, unlike the magnetars, anomalous X-ray pulsars
or soft γ repeaters that have repetitive flares of similar luminosities.
The SLUGGS Survey: revisiting the correlation between X-ray luminosity and
total mass of massive early-type galaxies
D.A. Forbes, A. Alabi, A.J. Romanowsky, D.-W. Kim, J.P. Brodie, G. Fabbiano
Monthly Notices Letters of the Royal Astronomical Society, Volume 464, Issue 1, 2017 January 01, pp. L26–L30
ADS |
MNRASL |
arXiv:1609.01713
Here we utilize recent measures of galaxy total dynamical mass and X-ray gas luminosities
(LX,Gas) for a sample of 29 massive early-type galaxies from the SLUGGS survey to probe
LX,Gas–mass scaling relations. In particular, we investigate scalings with stellar mass, dynamical
mass within 5 effective radii (Re) and total virial mass. We also compare these relations
with predictions from Λ cold dark matter simulations. We find a strong linear relationship
between LX,Gas and galaxy dynamical mass within 5Re, which is consistent with the recent
cosmological simulations of Choi et al. that incorporate mechanical heating from AGN. We
conclude that the gas surrounding massive early-type galaxies was shock-heated as it fell into
collapsing dark matter haloes so that LX,Gas is primarily driven by the depth of a galaxy's
potential well. Heating by an AGN plays an important secondary role in determining LX,Gas.
Metallicity and age of the stellar stream around the disk galaxy NGC 5907
S. Laine, C.J. Grillmair, P. Capak, R.G. Arendt, A.J. Romanowsky, D. Martínez-Delgado,
M.L.N. Ashby, J.E. Davies, S.R. Majewski, J.P. Brodie, R.J. GaBany, J.A. Arnold
The Astronomical Journal, Volume 152, Number 3, 2016 September, Article 72 (11pp)
ADS |
AJ |
arXiv:1607.03554
Press coverage:
AAS Nova
Stellar streams have become central to studies of the interaction histories of nearby galaxies.
To characterize the most prominent parts of the stellar stream around the well-known nearby
(d = 17 Mpc) edge-on disk galaxy NGC 5907, we have obtained and analyzed new, deep
gri Subaru/Suprime-Cam and 3.6 μm Spitzer/Infrared Array Camera
observations. Combining the near-infrared 3.6 μm data with visible-light images allows
us to use a long wavelength baseline to estimate the metallicity and age of the stellar
population along an ~60 kpc long segment of the stream. We have fitted the stellar spectral
energy distribution with a single-burst stellar population synthesis model and we use it to
distinguish between the proposed satellite accretion and minor/major merger formation models of
the stellar stream around this galaxy. We conclude that a massive minor merger (stellar mass
ratio of at least 1:8) can best account for the metallicity of –0.3 inferred along the
brightest parts of the stream.
A discrete chemo-dynamical model of the giant elliptical galaxy NGC 5846:
dark matter fraction, internal rotation and velocity anisotropy out
to six effective radii
L. Zhu, A.J. Romanowsky, G. van de Ven, R.J. Long, L.L Watkins,
V. Pota, N.R. Napolitano, D.A. Forbes, J. Brodie, C. Foster
Monthly Notices of the Royal Astronomical Society, Volume 462, Issue 4, 2016 November 11, pp. 4001–4017
ADS |
MNRAS |
arXiv:1608.08238
We construct a suite of discrete chemo-dynamical models of the giant elliptical galaxy NGC 5846. These models are a powerful
tool to constrain both the mass distribution and internal dynamics of multiple tracer populations. We use Jeans models to
simultaneously fit stellar kinematics within the effective radius Re, planetary nebula (PN) radial velocities out to
3 Re, and globular cluster (GC) radial velocities and colours out to 6 Re. The best-fitting
model is a cored dark matter halo which contributes ~10 per cent of the total mass within 1 Re, and
67 per cent ± 10 per cent within 6 Re, although a cusped dark matter halo is also acceptable. The red GCs
exhibit mild rotation with vmax/σ0 ~ 0.3 in the region
R > Re, aligned with but counter-rotating to the stars in the inner parts, while the blue GCs and PNe
kinematics are consistent with no rotation. The red GCs are tangentially anisotropic, the blue GCs are mildly radially
anisotropic, and the PNe vary from radially to tangentially anisotropic from the inner to the outer region. This is confirmed
by general made-to-measure models. The tangential anisotropy of the red GCs in the inner regions could stem from the
preferential destruction of red GCs on more radial orbits, while their outer tangential anisotropy – similar to the PNe
in this region – has no good explanation. The mild radial anisotropy of the blue GCs is consistent with an accretion
scenario.
A high stellar velocity dispersion and ~100 globular clusters in the
ultra-diffuse galaxy Dragonfly 44
P. van Dokkum, R. Abraham, J. Brodie, C. Conroy, S. Danieli, A. Merritt, L. Mowla,
A. Romanowsky, J. Zhang
The Astrophysical Journal Letters, Volume 828, Number 1, 2016 September 1, Article L6 (6pp)
ADS |
ApJL |
arXiv:1606.06291
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Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The
apparent survival of these ultra-diffuse galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here, we
present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the
Keck II telescope. We find a velocity dispersion of σ = 47+8–-6
km s–1, which implies a dynamical mass of Mdyn(< r1/2) =
0.7+0.3–0.2 × 1010 M☉ within its deprojected half-light radius
of r1/2 = 4.6 ± 0.2 kpc. The mass-to-light ratio is M/LI(< r1/2)
= 48+21–14 M☉/L☉, and the dark matter fraction is 98% within
r1/2. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini
imaging taken in 0.4" seeing we infer that Dragonfly 44 has 94+25–20 globular clusters, similar to
the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed"
galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the
total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r = 4.6 kpc to enclosed mass
profiles of NFW halos. The enclosed mass suggests a total mass of ~1012 M☉, similar to the mass of
the Milky Way. The existence of nearly dark objects with this mass is unexpected, as galaxy formation is thought to be
maximally efficient in this regime.
First results from the MADCASH Survey: a faint dwarf galaxy companion
to the low-mass spiral galaxy NGC 2403 at 3.2 Mpc
J.L. Carlin, D.J. Sand, P. Price, B. Willman, A. Karunakaran,
K. Spekkens, E.F. Bell, J.P. Brodie, Crnojević, D.A. Forbes, J. Hargis,
E.K. Kirby, R. Lupton, A.H.G. Peter, A.J. Romanowsky, J. Strader
The Astrophysical Journal Letters, Volume 828, Number 1, 2016 September 1, Article L5 (6pp)
ADS |
ApJL |
arXiv:1608.02591
We report the discovery of the faintest known dwarf galaxy satellite of a Large Magellanic Cloud (LMC) stellar-mass host
beyond the Local Group (LG), based on deep imaging with Subaru/Hyper Suprime-Cam. Magellanic Analog Dwarf Companions And
Stellar Halos (MADCASH) J074238+652501-dw lies ~35 kpc in projection from NGC 2403, a dwarf spiral galaxy at D
≈ 3.2 Mpc. This new dwarf has Mg = –7.4 ± 0.4 and a half-light radius of 168 ± 70 pc, at
the calculated distance of 3.39 ± 0.41 Mpc. The color–magnitude diagram reveals no evidence of young stellar
populations, suggesting that MADCASH J074238+652501-dw is an old, metal-poor dwarf similar to low-luminosity dwarfs in the LG.
The lack of either detected HI gas (MHI/LV < 0.69 M☉/L☉,
based on Green Bank Telescope observations) or GALEX NUV/FUV flux enhancement is consistent with a lack of young stars.
This is the first result from the MADCASH survey, which is conducting a census of the stellar substructure and faint satellites
in the halos of Local Volume LMC analogs via resolved stellar populations. Models predict a total of ~4–10 satellites at
least as massive as MADCASH J074238+652501-dw around a host with the mass of NGC 2403, with 2–3 within our field of view,
slightly more than the one such satellite observed in our footprint.
The SLUGGS survey: a new mask design to reconstruct the stellar populations and
kinematics of both inner and outer galaxy regions
N. Pastorello, D.A. Forbes, A. Poci, A.J. Romanowsky, R. McDermid,
A.B. Alabi, J.P. Brodie, M. Cappellari, V. Pota, C. Foster
Publications of the Astronomical Society of Australia, Volume 33, 2016, e035 (16 pp)
ADS |
PASA |
arXiv:1607.057991
Integral field unit spectrographs allow the 2D exploration of the kinematics and stellar populations of galaxies, although
they are generally restricted to small fields-of-view. Using the large field-of-view of the DEIMOS multislit spectrograph on
Keck and our Stellar Kinematics using Multiple Slits technique, we are able to extract sky-subtracted stellar light spectra to
large galactocentric radii. Here, we present a new DEIMOS mask design named SuperSKiMS that explores large spatial
scales without sacrificing high spatial sampling. We simulate a set of observations with such a mask design on the nearby
galaxy NGC 1023, measuring stellar kinematics and metallicities out to where the galaxy surface brightness is orders of
magnitude fainter than the sky. With this technique we also reproduce the results from literature integral field spectroscopy
in the innermost galaxy regions. In particular, we use the simulated NGC 1023 kinematics to model its total mass distribution
to large radii, obtaining comparable results with those from published integral field unit observation. Finally, from new
spectra of NGC 1023, we obtain stellar 2D kinematics and metallicity distributions that show good agreement with integral
field spectroscopy results in the overlapping regions. In particular, we do not find a significant offset between our Stellar
Kinematics using Multiple Slits and the ATLAS3D stellar velocity dispersion at the same spatial locations.
The mass discrepancy acceleration relation in early-type galaxies: extended mass profiles and the phantom menace to MOND
J. Janz, M. Cappellari, A.J. Romanowsky, L. Ciotti, A. Alabi, D.A. Forbes
Monthly Notices of the Royal Astronomical Society, Volume 461, Issue 3, 2016 September 21, pp. 2367–2373
MNRAS |
ADS |
arXiv:1606.05003
The dark matter (DM) haloes around spiral galaxies appear to conspire with
their baryonic content: empirically, significant amounts of DM are inferred
only below a universal characteristic acceleration scale. Moreover, the
discrepancy between the baryonic and dynamical mass, which is usually
interpreted as the presence of DM, follows a very tight mass discrepancy
acceleration (MDA) relation. Its universality, and its tightness in spiral
galaxies, poses a challenge for the DM interpretation and was used to argue
in favour of MOdified Newtonian Dynamics (MOND). Here, we test whether or
not this applies to early-type galaxies. We use the dynamical models of
fast-rotator early-type galaxies by Cappellari et al. based on
ATLAS3D and SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) data, which was the first homogenous study of this kind, reaching
~4 Re, where DM begins to dominate the total mass budget.
We find the early-type galaxies to follow an MDA relation similar to spiral
galaxies, but systematically offset. Also, while the slopes of the mass
density profiles inferred from galaxy dynamics show consistency with those
expected from their stellar content assuming MOND, some profiles of
individual galaxies show discrepancies.
The SLUGGS Survey: the mass distribution in early-type galaxies within five effective radii and beyond
A. Alabi, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader, J. Janz,
V. Pota, N. Pastorello, C. Usher, L.R. Spitler, C. Foster, Z.G. Jennings,
A. Villaume, S. Kartha
Monthly Notices of the Royal Astronomical Society, Volume 460, Issue 4, 2016 August 21, pp. 3838–3860
ADS |
MNRAS |
arXiv:1605.06101
We study mass distributions within and beyond 5 effective radii (Re) in 23 early-type
galaxies from the SAGES Legacy Unifying Globulars and Galaxies Survey, using their globular cluster (GC)
kinematic data. The data are obtained with Keck/DEep Imaging Multi-Object Spectrograph, and consist of
line-of-sight velocities for ~3500 GCs, measured with a high precision of ~15 km s–1 per
GC and extending out to ~13 Re. We obtain the mass distribution in each galaxy using the
tracer mass estimator of Watkins et al. and account for kinematic substructures, rotation of the GC systems
and galaxy flattening in our mass estimates. The observed scatter between our mass estimates and results
from the literature is less than 0.2 dex. The dark matter fraction within 5 Re
(fDM) increases from ~0.6 to ~0.8 for low- and high-mass galaxies, respectively, with
some intermediate-mass galaxies (M⭑ ~ 1011 M☉)
having low fDM ~ 0.3, which appears at odds with predictions from simple galaxy models.
We show that these results are independent of the adopted orbital anisotropy, stellar mass-to-light
(M/L) ratio, and the assumed slope of the gravitational potential. However, the low
fDM in the ~1011 M☉ galaxies agrees with the cosmological
simulations of Wu et al. where the pristine dark matter distribution has been modified by baryons during
the galaxy assembly process. We find hints that these M⭑ ~ 1011
M☉ galaxies with low fDM have very diffuse dark matter haloes, implying
that they assembled late. Beyond 5 Re, the M/L gradients are steeper in the
more massive galaxies and shallower in both low and intermediate mass galaxies.
Star clusters in M31: VII. Global kinematics and metallicity subpopulations of the globular clusters
N. Caldwell, A.J. Romanowsky
The Astrophysical Journal, Volume 824, Number 1, 2016 June 10, Article 42 (8pp)
ADS |
ApJ |
arXiv:1603.06947
We carry out a joint spatial–kinematical–metallicity analysis of globular clusters (GCs)
around the Andromeda Galaxy (M31), using a homogeneous, high-quality spectroscopic data set. In particular, we remove the contaminating young clusters that have plagued many previous analyses. We find that the
clusters can be divided into three major metallicity groups based on their radial distributions: (1) an
inner metal-rich group ([Fe/H] > –0.4); (2) a group with intermediate metallicity (with median [Fe/H]
= –1); and (3) a metal-poor group, with [Fe/H] < –1.5. The metal-rich group has kinematics and
spatial properties like those of the disk of M31, while the two more metal-poor groups show mild prograde
rotation overall, with larger dispersions—in contrast to previous claims of stronger rotation. The
metal-poor GCs are the least concentrated group; such clusters occur five times less frequently in the
central bulge than do clusters of higher metallicity. Despite some well-known differences between the M31
and Milky Way GC systems, our revised analysis points to remarkable similarities in their chemodynamical
properties, which could help elucidate the different formation stages of galaxies and their GCs. In
particular, the M31 results motivate further exploration of a metal-rich GC formation mode in situ, within
high-redshift, clumpy galactic disks.
New spectroscopic technique based on coaddition of surface brightness fluctuations:
NGC4449 and its stellar tidal stream
E. Toloba, P. Guhathakurta, A.J. Romanowsky, J.P. Brodie, D. Martínez-Delgado, J.A. Arnold, N. Ramachandran, K. Theakanath
The Astrophysical Journal, Volume 824, Number 1, 2016 June 10, Article 35 (11pp)
ADS |
ApJ |
arXiv:1605.03967
We present a new spectroscopic technique based in part on targeting the upward fluctuations of the surface
brightness for studying the internal stellar kinematics and metallicities of galaxies of low surface
brightness effects both to galaxies and streams beyond the Local Group. The distance to these systems
makes them unsuitable for targeting individual red giant branch (RGB) stars (tip of RGB at I ≳ 24
mag) and their surface brightness is too low (μr ≳ 25 mag arcsec–2)
for integrated light spectroscopic measurements. This technique overcomes these two problems by targeting
individual objects that are brighter than the tip of the RGB. We apply this technique to the star-forming
dwarf galaxy NGC 4449 and its stellar stream. We use Keck/DEIMOS data to measure the line-of-sight radial
velocity out to ~7 kpc in the east side of the galaxy and ~8 kpc along the stream. We find that the two
systems are likely gravitationally bound to each other and have heliocentric radial velocities of 227.3
± 10.7 km s–1 and 225.8 ± 16.0 km s–1, respectively. Neither the
stream nor the near half of the galaxy shows a significant velocity gradient. We estimate the stellar
metallicity of the stream based on the equivalent width of its calcium triplet lines and find [Fe/H] =
–1.37 ± 0.41, which is consistent with the metallicity–luminosity relation for dwarf
galaxies in the Local Group. Whether the stream's progenitor was moderately or severely stripped cannot be
constrained with this uncertainty in metallicity. We demonstrate that this new technique can be used to
measure the kinematics and (possibly) the metallicity of the numerous faint satellites and stellar streams
in the halos of nearby (~4 Mpc) galaxies.
Discovery of the candidate off-nuclear ultrasoft hyper-luminous X-ray source 3XMM J141711.1+522541
D. Lin, E.R. Carrasco, N.A. Webb, J.A. Irwin, R. Dupke, A.J. Romanowsky, E. Ramirez-Ruiz, J. Strader, J. Homan, D. Barret, O. Godet
The Astrophysical Journal, Volume 821, Number 1, 2016 April 10, Article 25 (12pp)
ADS |
ApJ |
arXiv:1603.00455
We report the discovery of an off-nuclear ultrasoft hyper-luminous X-ray source candidate 3XMM
J141711.1+522541 in the inactive S0 galaxy SDSS J141711.07+522540.8 (z = 0.41827,
dL = 2.3 Gpc) in the Extended Groth Strip. It is located at a projected offset of
~1.0" (5.2 kpc) from the nucleus of the galaxy and was serendipitously detected in five XMM-Newton
observations in 2000 July. Two observations have enough counts and can be fitted with a standard
thermal disk with an apparent inner disk temperature kTMCD ~ 0.13 keV and a
0.28–14.2 keV unabsorbed luminosity LX ~ 4×1043
erg s–1 in the source rest frame. The source was still detected in three
Chandra observations in 2002 August, with similarly ultrasoft but fainter spectra
(kTMCD ~ 0.17 keV, LX ~ 0.5×1043 erg
s–1). It was not detected in later observations, including two by Chandra
in 2005 October, one by XMM-Newton in 2014 January, and two by Chandra in 2014
September–October, implying a long-term flux variation factor of >14. Therefore the source
could be a transient with an outburst in 2000–2002. It has a faint optical counterpart
candidate, with apparent magnitudes of mF606W = 26.3 AB mag and
mF814W = 25.5 AB mag in 2004 December (implying an absolute V-band
magnitude of ~ –15.9 AB mag). We discuss various explanations for the source and find that it
is best explained as a massive black hole (BH) embedded in the nucleus of a possibly stripped
satellite galaxy, with the X-ray outburst due to tidal disruption of a surrounding star by the BH.
The BH mass is ~105 M☉, assuming the peak X-ray luminosity at
around the Eddington limit.
Discovery of an ultra-diffuse galaxy in the Pisces–Perseus supercluster
D. Martínez-Delgado, R. Läsker, M. Sharina, E. Toloba, J. Fliri, R. Beaton, D. Valls-Gabaud, I.D. Karachentsev, T.S. Chonis, E.K. Grebel,
D.A. Forbes, A.J. Romanowsky, J. Gallego-Laborda, K. Teuwen,
M.A. Gómez-Flechoso, J. Wang, P. Guhathakurta, S. Kaisin, N. Ho
The Astronomical Journal, Volume 151, Number 4, 2016 April, Article 96 (13pp)
AJ |
ADS |
arXiv:1601.06960
We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located
10.4° in projection from the Andromeda
galaxy (M31). This low-surface brightness galaxy (μV = 24.8 mag
arcsec–2), found with a small amateur telescope, appears unresolved
in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by
optical surveys relying on resolved star counts, in spite of its relatively large
effective radius (Re(V) = 12") and proximity (15') to the
well-known dwarf spheroidal galaxy And II. Its red color (V–I = 1.0),
shallow Sérsic index (nV = 0.68), and the absence of detectable
Hα emission are typical properties of dwarf spheroidal galaxies and
suggest that it is mainly composed of old stars. Initially interpreted as an
interesting case of an isolated dwarf spheroidal galaxy in the local universe, our
radial velocity measurement obtained with the BTA 6 m telescope (Vh
= 5450 ± 40 km s–1) shows that this system is an M31-background
galaxy associated with the filament of the Pisces-Perseus supercluster. At the
distance of this cluster (~78 Mpc), DGSAT I would have an Re ~ 4.7
kpc and MV –16.3. Its properties resemble those of the
ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the
first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated
with the Coma and Virgo clusters, DGSAT I is found in a much lower density
environment, which provides a fresh constraint on the formation mechanisms for this
intriguing class of galaxy.
The SLUGGS survey: exploring the globular cluster systems of the Leo II group and their global relationships
S.S. Kartha, D.A. Forbes, A.B. Alabi, J.P. Brodie, A.J. Romanowsky,
J. Strader, L.R. Spitler, Z.G. Jennings, J.C. Roediger
Monthly Notices of the Royal Astronomical Society, Volume 458, Issue 1, 2016 May 01, pp. 105–126
ADS |
MNRAS |
arXiv:1602.01838
We present an investigation of the globular cluster (GC) systems of NGC 3607 and
NGC 3608 as part of the ongoing SLUGGS (SAGES Legacy Unifying Globulars and
GalaxieS) survey. We use wide-field imaging data from the Subaru telescope in the
g, r and i filters to analyse the radial density, colour and
azimuthal distributions of both GC systems. With the complementary kinematic data
obtained from the Keck II telescope, we measure the radial velocities of a total
of 81 GCs. Our results show that the GC systems of NGC 3607 and NGC 3608 have a
detectable spatial extent of ~15 and 13 galaxy effective radii, respectively. Both
GC systems show a clear bimodal colour distribution. We detect a significant radial
colour gradient for the GC subpopulations in both galaxies. NGC 3607 exhibits an
overabundance of red GCs on the galaxy minor axis and NGC 3608 shows a misalignment
in the GC subpopulation position angles with respect to the galaxy stellar component.
With the aid of literature data, we discuss several relationships between the
properties of GC systems and their host galaxies. A one-to-one relation between the
ellipticities of red GCs and the galaxy stellar light emphasizes the evolutionary
similarities between them. In our sample of four slowly rotating galaxies with
kinematically decoupled cores, we observe a higher ellipticity for the blue GC
subpopulation than their red counterparts. Also, we notice the flattening of
negative colour gradients for the blue GC subpopulations with increasing galaxy
stellar mass. Finally, we discuss the formation scenarios associated with the blue
GC subpopulation.
An overmassive dark halo around an ultra-diffuse galaxy in the Virgo cluster
M.A. Beasley, A.J. Romanowsky, V. Pota, I. Martin-Navarro, D. Martinez-Delgado, F. Neyer, A.L. Deich
The Astrophysical Journal Letters, Volume 819, Number 2, 2016 March 10, Article L20 (7pp)
ADS |
ApJL |
arXiv:1602.04002
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Ultra-diffuse galaxies (UDGs) have the sizes of giants but the luminosities of dwarfs. A key to
understanding their origins comes from their total masses, but their low surface brightnesses
(μ(V) ≥ 25.0) generally prohibit dynamical studies. Here, we report the
first such measurements for a UDG (VCC 1287 in the Virgo cluster), based on its globular cluster
system dynamics and size. From seven GCs we measure a mean systemic velocity
vsys = 1071+14–15 km s–1, thereby confirming a Virgo
cluster association. We measure a velocity dispersion of 33+16–10
km s–1 within 8.1 kpc, corresponding to an enclosed mass of (4.5 ± 2.8) ×
109 M☉ and a g-band mass-to-light ratio of
(M/L)g = 106+126–54 within an effective radius. From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of ~8
×1010 M☉, yielding a dark-to-stellar mass fraction of ~3000. We show
that this UDG is an outlier in Mstar–Mhalo relations,
suggesting extreme stochasticity in relatively massive star-forming halos in clusters. Finally, we
discuss how counting GCs offers an efficient route to determining virial masses for UDGs.
The SLUGGS survey:
globular clusters and the dark matter content of early-type galaxies
D.A. Forbes, A. Alabi, A.J. Romanowsky, J.P. Brodie, J. Strader, C. Usher, V. Pota
Monthly Notices Letters of the Royal Astronomical Society, Volume 458, Issue 1, 2016 May 01, pp. L44–L48
ADS |
MNRAS |
arXiv:1602.01105
A strong correlation exists between the total mass of a globular cluster (GC) system and the
virial halo mass of the host galaxy. However, the total halo mass in this correlation is a
statistical measure conducted on spatial scales that are some 10 times that of a typical GC
system. Here we investigate the connection between GC systems and galaxy's dark matter on
comparable spatial scales, using dynamical masses measured on a galaxy-by-galaxy basis. Our
sample consists of 17 well-studied massive (~1011 M☉) early-type
galaxies from the SLUGGS survey. We find the strongest correlation to be that of the blue
(metal-poor) GC subpopulation and the dark matter content. This correlation implies that the
dark matter mass of a galaxy can be estimated to within a factor of 2 from careful imaging
of its GC system. The ratio of the GC system mass to that of the enclosed dark matter is
nearly constant. We also find a strong correlation between the fraction of blue GCs and the
fraction of enclosed dark matter, so that a typical galaxy with a blue GC fraction of 60 per
cent has a dark matter fraction of 86 per cent over similar spatial scales. Both halo growth
and removal (via tidal stripping) may play some role in shaping this trend. In the context
of the two-phase model for galaxy formation, we find galaxies with the highest fractions of
accreted stars to have higher dark matter fractions for a given fraction of blue GCs.
The SLUGGS Survey: the assembly histories of individual early-type galaxies
D.A. Forbes, A.J. Romanowsky, N. Pastorello, C. Foster,
J.P. Brodie, J. Strader, C. Usher, V. Pota
Monthly Notices of the Royal Astronomical Society, Volume 457, Issue 2, 2016 April 01, pp. 1242–1256
ADS |
MNRAS |
arXiv:1601.02597
Early-type (E and S0) galaxies may have assembled via a variety of different
evolutionary pathways. Here, we investigate these pathways by comparing the
stellar kinematic properties of 24 early-type galaxies from the SAGES Legacy
Unifying Globulars and GalaxieS (SLUGGS) survey with the hydrodynamical
simulations of Naab et al. In particular, we use the kinematics of starlight
up to 4 effective radii (Re) as diagnostics of galaxy inner
and outer regions, and assign each galaxy to one of six Naab et al. assembly
classes. The majority of our galaxies (14/24) have kinematic characteristics
that indicate an assembly history dominated by gradual gas dissipation and
accretion of many gas-rich minor mergers. Three galaxies, all S0s, indicate
that they have experienced gas-rich major mergers in their more recent past.
One additional elliptical galaxy is tentatively associated with a gas-rich
merger which results in a remnant galaxy with low angular momentum. Pathways
dominated by gas-poor (major or minor) mergers dominate the mass growth of six
galaxies. Most SLUGGS galaxies appear to have grown in mass (and size) via the
accretion of stars and gas from minor mergers, with late major mergers playing
a much smaller role. We find that the fraction of accreted stars correlates
with the stellar mean age and metallicity gradient, but not with the slope of
the total mass density profile. We briefly mention future observational and
modelling approaches that will enhance our ability to accurately reconstruct
the assembly histories of individual present-day galaxies.
Satellite accretion in action: a tidally disrupting dwarf
spheroidal around the nearby spiral galaxy NGC 253
A.J. Romanowsky, D. Martínez-Delgado, N.F. Martin,
G. Morales, Z.G. Jennings, R.J. GaBany, J.P. Brodie, E.K. Grebel,
J. Schedler, M. Sidonio
Monthly Notices Letters of the Royal Astronomical Society, Volume 457, Issue 1, 2016 March 21, pp. L103–L107
ADS |
MNRAS
(free) |
arXiv:1512.03815
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We report the discovery of NGC 253-dw2, a dwarf spheroidal (dSph) galaxy candidate undergoing
tidal disruption around a nearby spiral galaxy, NGC 253 in the Sculptor group: the first such
event identified beyond the Local Group. The dwarf was found using small-aperture amateur
telescopes, and followed up with Suprime-Cam on the 8 m Subaru Telescope in order to resolve
its brightest stars. Using g- and RC-band photometry, we detect a red
giant branch consistent with an old, metal-poor stellar population at a distance of ~3.5 Mpc.
From the distribution of likely member stars, we infer a highly elongated shape with a
semimajor axis half-light radius of (2 ± 0.4) kpc. Star counts also yield a luminosity
estimate of ~2×106 L☉,V (MV
~ –10.7). The morphological properties of NGC 253-dw2 mark it as distinct from normal
dSphs and imply ongoing disruption at a projected distance of ~50 kpc from the main galaxy.
Our observations support the hierarchical paradigm wherein massive galaxies continuously
accrete less massive ones, and provide a new case study for dSph infall and dissolution
dynamics. We also note the continued efficacy of small telescopes for making big discoveries.
The SLUGGS Survey: stellar kinematics, kinemetry and trends at large radii in 25 early-type galaxies
C. Foster, N. Pastorello, J. Roediger, J.P. Brodie, D.A. Forbes, S.S. Kartha, V. Pota, A.J. Romanowsky,
L.R. Spitler, J. Strader, C. Usher, J.A. Arnold
Monthly Notices of the Royal Astronomical Society, Volume 457, Issue 1, 2016 March 21, pp. 147–171
ADS |
MNRAS |
arXiv:1512.06130
Due to longer dynamical time-scales, the outskirts of early-type galaxies retain the footprint
of their formation and assembly. Under the popular two-phase galaxy formation scenario, an
initial in situ phase of star formation is followed by minor merging and accretion of ex
situ stars leading to the expectation of observable transitions in the kinematics and stellar
populations on large scales. However, observing the faint galactic outskirts is challenging,
often leaving the transition unexplored. The large-scale, spatially resolved stellar kinematic
data from the SAGES Legacy Unifying Galaxies and GlobularS (SLUGGS) survey are ideal for
detecting kinematic transitions. We present kinematic maps out to 2.6 effective radii on
average, kinemetry profiles, measurement of kinematic twists and misalignments, and the average
outer intrinsic shape of 25 SLUGGS galaxies. We find good overall agreement in the kinematic
maps and kinemetry radial profiles with literature. We are able to confirm significant radial
modulations in rotational versus pressure support of galaxies with radius so that the central
and outer rotational properties may be quite different. We also test the suggestion that galaxies
may be more triaxial in their outskirts and find that while fast rotating galaxies were already
shown to be axisymmetric in their inner regions, we are unable to rule out triaxiality in their
outskirts. We compare our derived outer kinematic information to model predictions from a
two-phase galaxy formation scenario. We find that the theoretical range of local outer angular
momentum agrees well with our observations, but that radial modulations are much smaller than
predicted.
The SLUGGS survey: chromodynamical modelling of the lenticular galaxy NGC 1023
A. Cortesi, A.L. Chies-Santos, V. Pota, C. Foster, L. Coccato,
C. Mendes de Oliveira, D.A. Forbes, M.M. Merrifield, S.P. Bamford,
A.J. Romanowsky, J.P. Brodie, S.S. Kartha, A.B. Alabi,
R.N. Proctor, A. Almeida
Monthly Notices of the Royal Astronomical Society, Volume 456, Issue 3, 2016 March 01, pp. 2611–2621
ADS |
MNRAS |
arXiv:1512.01185
Globular clusters (GCs) can be considered discrete, long-lived, dynamical
tracers that retain crucial information about the assembly history of their
parent galaxy. In this paper, we present a new catalogue of GC velocities and
colours for the lenticular galaxy NGC 1023, we study their kinematics and
spatial distribution, in comparison with the underlying stellar kinematics
and surface brightness profile, and we test a new method for studying GC
properties. Specifically, we decompose the galaxy light into its spheroid
(assumed to represent the bulge+halo components) and disc components and use
it to assign to each GC a probability of belonging to one of the two
components. Then we model the galaxy kinematics, assuming a disc and
spheroidal component, using planetary nebulae and integrated stellar light.
We use this kinematic model and the probability previously obtained from the
photometry to recalculate for each GC its likelihood of being associated with
the disc, the spheroid, or neither. We find that the reddest GCs are likely
to be associated with the disc, as found for faint fuzzies in this same
galaxy, suggesting that the disc of this S0 galaxy originated at
z ≅ 2. The majority of blue GCs are found likely to be associated
with the spheroidal (hot) component. The method also allows us to identify
objects that are unlikely to be in equilibrium with the system. In NGC 1023
some of the rejected GCs form a substructure in phase space that is connected
with NGC 1023 companion galaxy.
The AIMSS Project – III. The stellar populations of compact stellar systems
J. Janz, M.A. Norris, D.A. Forbes, A. Huxor, A.J. Romanowsky, M.J. Frank,
C.G. Escudero, F.R. Faifer, J.C. Forte, S.J. Kannappan, C. Maraston,
J.P. Brodie, J. Strader, B.R. Thompson
Monthly Notices of the Royal Astronomical Society, Volume 456, Issue 1, 2016 February 11, pp. 617–632
ADS |
MNRAS |
arXiv:1511.03264
In recent years, a growing zoo of compact stellar systems (CSSs) have been
found whose physical properties (mass, size, velocity dispersion) place them
between classical globular clusters (GCs) and true galaxies, leading to
debates about their nature. Here we present results using a so far
underutilized discriminant, their stellar population properties. Based on new
spectroscopy from 8–10m telescopes, we derive ages, metallicities, and
[α/Fe] of 29 CSSs. These range from GCs with sizes of merely a few
parsec to compact ellipticals (cEs) larger than M32. Together with a
literature compilation, this provides a panoramic view of the stellar
population characteristics of early-type systems. We find that the CSSs are
predominantly more metal rich than typical galaxies at the same stellar mass.
At high mass, the cEs depart from the mass–metallicity relation of
massive early-type galaxies, which forms a continuous sequence with dwarf
galaxies. At lower mass, the metallicity distribution of ultracompact dwarfs
(UCDs) changes at a few times 107 M☉, which roughly
coincides with the mass where luminosity function arguments previously
suggested the GC population ends. The highest metallicities in CSSs are
paralleled only by those of dwarf galaxy nuclei and the central parts of
massive early types. These findings can be interpreted as CSSs previously
being more massive and undergoing tidal interactions to obtain their
current mass and compact size. Such an interpretation is supported by CSSs
with direct evidence for tidal stripping, and by an examination of the CSS
internal escape velocities.
NGC 3628-UCD1: a possible ω Cen analog embedded in a stellar stream
Z.G. Jennings, A.J. Romanowsky, J.P. Brodie, J. Janz, M.A. Norris, D.A. Forbes, D. Martinez-Delgado, M. Fagioli, S.J. Penny
The Astrophysical Journal Letters, Volume 812, Number 1, 2015 October 10, Article L10 (6pp)
ApJL |
ADS |
arXiv:1509.04710
Using Subaru/Suprime-Cam wide-field imaging and both Keck/ESI and LBT/MODS spectroscopy, we
identify and characterize a compact star cluster, which we term NGC 3628-UCD1, embedded in a
stellar stream around the spiral galaxy NGC 3628. The size and luminosity of UCD1 are similar to
ω Cen, the most luminous Milky Way globular cluster, which has long been suspected to
be the stripped remnant of an accreted dwarf galaxy. The object has a magnitude of
i = 19.3 mag (Li = 1.4 × 106
L☉).
UCD1 is marginally resolved in our ground-based imaging, with a half-light radius of ~10 pc.
We measure an integrated brightness for the stellar stream of i = 13.1 mag, with
(g–i) = 1.0. This would correspond an accreted dwarf galaxy with an
approximate luminosity of Li ~ 4.1 × 108
L☉. Spectral analysis reveals that UCD1 has an age of 6.6 Gyr,
[Z/H] = –0.75, and [α/Fe] = –0.10. We propose that UCD1 is an
example of an ω Cen-like star cluster possibly forming from the nucleus of an
infalling dwarf galaxy, demonstrating that at least some of the massive star cluster population
may be created through tidal stripping.
VEGAS: A VST Early-type GAlaxy Survey. I. Presentation, wide-field surface photometry,
and substructures in NGC 4472
M. Capaccioli, M. Spavone, A. Grado, E. Iodice, L. Limatola, N.R. Napolitano, M. Cantiello, M. Paolillo, A.J. Romanowsky, D.A. Forbes, T.H. Puzia, G. Raimondo, P. Schipani
Astronomy & Astrophysics, Volume 581, September 2015, Article A10 (35pp)
ADS |
A&A |
arXiv:1507.01336
We present the VST Early-type GAlaxy Survey (VEGAS), which is designed to
obtain deep multiband photometry in g,r,i,
of about one hundred nearby
galaxies down to 27.3, 26.8, and 26 mag/arcsec2 respectively,
using the ESO facility VST/OmegaCAM.
The goals of the survey are 1) to map the light distribution up to ten
effective radii, re; 2) to trace color gradients and
surface brightness fluctuation gradients out to a few re
for stellar population characterization; and 3) to obtain a full census of
the satellite systems (globular clusters and dwarf galaxies) out to 20% of
the galaxy virial radius. The external regions of galaxies retain signatures
of the formation and evolution mechanisms that shaped them, and the study of
nearby objects enables a detailed analysis of their morphology and
interaction features. To clarify the complex variety of formation mechanisms
of early-type galaxies (ETGs), wide and deep photometry is the primary
observational step, which at the moment has been pursued with only a few
dedicated programs. The VEGAS survey has been designated to provide these
data for a volume-limited sample with exceptional image quality.
In this commissioning photometric paper we illustrate the capabilities of
the survey using g- and i-band VST/OmegaCAM images of the
nearby galaxy NGC 4472 and of smaller ETGs in the surrounding field.
Our surface brightness profiles reach rather faint levels and agree
excellently well with previous literature. Genuine new results concern the
detection of an intracluster light tail in NGC 4472 and of various
substructures at increasing scales. We have also produced extended
(g–i) color profiles.
The VST/OmegaCAM data that we acquire in the context of the VEGAS survey
provide a detailed view of substructures in the optical emission from
extended galaxies, which can be as faint as a hundred times below the
sky level.
The SLUGGS survey: globular cluster kinematics in a
'double sigma' galaxy – NGC 4473
A. Alabi, C. Foster, D.A. Forbes, A.J. Romanowsky, N. Pastorello,
J.P. Brodie, L.R. Spitler, J. Strader, C. Usher
Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 2, 2015 September 11, pp. 2208–2219
ADS |
MNRAS |
arXiv:1506.07882
NGC 4473 is a so-called double sigma (2σ) galaxy, i.e.
a galaxy with rare, double peaks in its 2D stellar velocity dispersion. Here, we present the globular cluster (GC) kinematics in NGC 4473 out to
~10 Re (effective radii) using data from combined Hubble
Space Telescope/Advanced Camera for Surveys and Subaru/Suprime-Cam imaging
and Keck/Deep Imaging Multi-Object Spectrograph. We find that the 2σ
nature of NGC 4473 persists up to 3 Re, though it becomes
misaligned to the photometric major axis. We also observe a significant offset
between the stellar and GC rotation amplitudes. This offset can be understood
as a co-addition of counter-rotating stars producing little net stellar
rotation. We identify a sharp radial transition in the GC kinematics at
~4 Re suggesting a well defined kinematically distinct halo.
In the inner region (<4 Re), the blue GCs rotate along the
photometric major axis, but in an opposite direction to the galaxy stars and
red GCs. In the outer region (>4 Re), the red GCs rotate in
an opposite direction compared to the inner region red GCs, along the
photometric major axis, while the blue GCs rotate along an axis intermediate
between the major and minor photometric axes. We also find a kinematically
distinct population of very red GCs in the inner region with elevated rotation
amplitude and velocity dispersion. The multiple kinematic components in NGC
4473 highlight the complex formation and evolutionary history of this
2σ galaxy, as well as a distinct transition between the inner
and outer components.
Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey
M.A. Sandoval, R.P. Vo, A.J. Romanowsky, J. Strader, J. Choi, Z.G. Jennings, C. Conroy, J.P. Brodie, C. Foster, A. Villaume, M.A. Norris, J. Janz, D.A. Forbes
The Astrophysical Journal Letters, Volume 808, Number 1, 2015 July 20, Article L32 (7pp)
ADS |
ApJL |
arXiv:1506.08828
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Motivated by the recent, serendipitous discovery of the densest known galaxy, M60-UCD1, we present
two initial findings from a follow-up search, using the Sloan Digital Sky Survey, Subaru/Suprime-Cam
and Hubble Space Telescope imaging, and SOuthern Astrophysics Research (SOAR)/Goodman spectroscopy.
The first object discovered, M59-UCD3, has a similar size to M60-UCD1 (half-light radius of rh ~ 20 pc) but
is 40% more luminous (MV ~ –14.6),
making it the new densest-known galaxy.
The second, M85-HCC1, has a size like a typical globular cluster
(GC; rh ~ 1.8 pc) but is much more luminous
(MV ~ –12.5).
This hypercompact cluster is by far the densest confirmed free-floating stellar system, and is equivalent to the densest known nuclear star clusters.
From spectroscopy, we find that both objects are
relatively young (~ 9 Gyr and ~ 3 Gyr, respectively), with metal-abundances that resemble those of galaxy centers.
Their host galaxies show clear signs of large-scale disturbances, and we conclude that these dense objects are the
remnant nuclei of recently accreted galaxies.
M59-UCD3 is an ideal target for follow-up with high-resolution imaging and spectroscopy to
search for an overweight central supermassive black hole as was discovered in M60-UCD1.
These findings also emphasize the potential value of
ultra-compact dwarfs and massive GCs as tracers of the assembly histories of galaxies.
The SLUGGS survey: inferring the formation epochs of metal-poor and metal-rich globular clusters
D.A. Forbes, N. Pastorello, A. Romanowsky, C. Usher, J.P. Brodie, J. Strader
Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 1, 2015 September 01, pp. 1045–1051
ADS |
MNRAS |
arXiv:1506.06820
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We present a novel, observationally-based framework for the formation
epochs and sites of globular clusters (GCs) in a cosmological context. Measuring directly the mean ages of the metal-poor and metal-rich GC
subpopulations in our own Galaxy, and in other galaxies, is
observationally challenging. Here we apply an alternative approach
utilizing the property that the galaxy mass–metallicity relation
is a strong function of redshift (or look-back age) but is relatively
insensitive to galaxy mass for massive galaxies. Assuming that GCs
follow galaxy mass–metallicity relations that evolve with redshift,
one can estimate the mean formation epochs of the two GC subpopulations
by knowing their mean metallicities and the growth in host galaxy mass
with redshift. Recently, the SAGES Legacy Unifying Globulars and GalaxieS
(SLUGGS) survey has measured the spectroscopic metallicities for over
1000 GCs in a dozen massive early-type galaxies. Here we use these
measurements, and our new metallicity matching method, to infer a mean
age for metal-rich GCs of 11.5 Gyr (z = 2.9) and a range of
12.2–12.8 Gyr (4.8 < z < 5.9) for the metal-poor GCs,
depending on whether they mostly formed in accreted satellites or
in situ within the main host galaxy. We compare our values to
direct age measurements for Milky Way GCs and predictions from
cosmological models. Our findings suggest that reionization preceded
most GC formation, and that it is unlikely to be the cause of GC
bimodal metallicity distributions.
The megasecond Chandra X-ray visionary project observation of NGC 3115 (III): luminosity functions of LMXBs and dependence on stellar environments
D. Lin, J.A. Irwin, K.-W. Wong, Z.G. Jennings, J. Homan, A.J. Romanowsky, J. Strader, J.P. Brodie, G.R. Sivakoff, R.A. Remillard
The Astrophysical Journal, Volume 808, Number 1, 2015 July 20, Article 20 (11pp)
ADS |
ApJ |
arXiv:1506.05447
We studied the X-ray luminosity function (XLF) of low-mass X-ray binaries
(LMXBs) in the nearby lenticular galaxy NGC 3115, using the Megasecond
Chandra X-ray Visionary Project Observation. With a total exposure
time of ~1.1 Ms, we constructed the XLF down to a limiting luminosity of
~1036 erg s–1, which is much deeper than that
typically reached for other early-type galaxies. We found significant
flattening of the overall LMXB XLF from dN/dL ∝
L–2.2 ± 0.4 above 5.5 ×
1037 erg s–1 to dN/dL ∝
L–1.0 ± 0.1 below it, although we could
not rule out a fit with a higher break at ~1.6 × 1038
erg s–1. We also found evidence that the XLF of LMXBs
in globular clusters (GCs) is overall flatter than that of field LMXBs.
Thus, our results for this galaxy do not support the idea that all LMXBs
are formed in GCs. The XLF of field LMXBs seems to show spatial variation,
with the XLF in the inner region of the galaxy being flatter than that
in the outer region, probably due to contamination of LMXBs from
undetected and/or disrupted GCs in the inner region. The XLF in the outer
region is probably the XLF of primordial field LMXBs, exhibiting
dN/dL ∝ L–1.2 ± 0.1
up to a break close to the Eddington limit of neutron star LMXBs
(~1.7 × 1038 erg s–1). The break of
the GC LMXB XLF is lower, at ~1.1 × 1037 erg
s–1. We also confirm previous findings that the
metal-rich/red GCs are more likely to host LMXBs than the metal-poor/blue
GCs, which is more significant for more luminous LMXBs, and that more
massive GCs are more likely to host LMXBs.
The megasecond Chandra X-ray visionary project observation of NGC 3115 (II): properties of point sources
D. Lin, J.A. Irwin, K.-W. Wong, Z.G. Jennings, J. Homan, A.J. Romanowsky, J. Strader, G.R. Sivakoff, J.P. Brodie, R.A. Remillard
The Astrophysical Journal, Volume 808, Number 1, 2015 July 20, Article 19 (21pp)
ADS |
ApJ |
arXiv:1506.05448
We carried out an in-depth study of low-mass X-ray binaries (LMXBs)
detected in the nearby lenticular galaxy NGC 3115 using the Megasecond
Chandra X-ray Visionary Project observation (total exposure time
1.1 Ms). In total we found 136 candidate LMXBs in the field and 49 in globular clusters (GCs) above 2σ detection,
with 0.3–8 keV luminosity
LX ~ 1036–1039 erg
s–1. Other than 13 transient candidates, the sources
overall have less long-term variability at higher luminosity, at least
at LX ≳ 1037 erg s–1.
In order to identify the nature and spectral
state of our sources, we compared their collective spectral properties
based on single-component models (a simple power law or a multicolor
disk) with the spectral evolution seen in representative Galactic LMXBs.
We found that in the LX versus photon index
ΓPL and
LX versus disk temperature kTMCD
plots, most of our
sources fall on a narrow track in which the spectral shape hardens with
increasing luminosity below LX ~ 7 ×
1037 erg s–1, but is relatively
constant (ΓPL ~ 1.5 or kTMCD
~ 1.5 keV) above this luminosity, which is similar to the spectral
evolution of Galactic neutron star (NS) LMXBs in the soft state in the
Chandra bandpass. Therefore, we identified the track as the NS
LMXB soft-state track and suggested sources with LX
≲ 7 × 1037 erg s–1 as atolls in
the soft state and those with LX ≳ 7 ×
1037 erg s–1 as Z sources. Ten other
sources (five are transients) displayed significantly softer spectra
and are probably black hole X-ray binaries in the thermal state. One of
them (persistent) is in a metal-poor GC.
VIMOS mosaic integral-field spectroscopy of the bulge and disc of the early-type galaxy NGC 4697
C. Spiniello, N.R. Napolitano, L. Coccato, V. Pota, A.J. Romanowsky, C. Tortora, G. Covone, M. Capaccioli
Monthly Notices of the Royal Astronomical Society, Volume 452, Issue 1, 2015 September 01, pp. 99–114
ADS |
MNRAS |
arXiv:1506.01037
We present an integral-field study of the internal structure, kinematics and
stellar population of the almost edge-on, intermediate-luminosity
(L⭑) elliptical galaxy NGC 4697. We build extended
two-dimensional (2D) maps of the stellar kinematics and line strengths of the
galaxy up to ~0.7 effective radii (Reff) using a mosaic of
eight VIMOS (VIsible Multi-Objects Spectrograph, on the Very Large Telescope)
integral-field unit pointings. We find clear evidence for a rotation-supported
structure along the major axis from the 2D kinematical maps, confirming the
previous classification of this system as a 'fast rotator'. We study the
correlations between the third and fourth Gauss–Hermite moments of the
line-of-sight velocity distribution (LOSVD) h3 and
h4 with the rotation parameter (V/σ), and
compare our findings to hydrodynamical simulations. We find remarkable
similarities to predictions from gas-rich mergers. Based on photometry, we
perform a bulge/disc decomposition and study the stellar population properties
of the two components. The bulge and the disc show different stellar populations,
with the stars in the bulge being older
(agebulge = 13.5+1.4–1.4 Gyr,
agedisc = 10.5+1.6–2.0 Gyr) and more metal
poor ([M/H]bulge = –0.17+0.12–0.1,
[M/H]disc = –0.03+0.02–0.1).
The evidence of a later-formed, more metal-rich disc embedded in an older, more
metal poor bulge, together with the LOSVD structure, supports a mass assembly
scenario dominated by gas-rich minor mergers and possibly with a late gas-rich
major merger that left a previously rapidly rotating system unchanged. The bulge
and the disc do not show signs of different stellar initial mass function (IMF)
slopes, and both match well with a Milky Way-like IMF.
The SLUGGS survey: combining stellar and globular cluster
metallicities in the outer regions of early-type galaxies
N. Pastorello, D.A. Forbes, C. Usher, J.P. Brodie, A.J. Romanowsky,
J. Strader, L.R. Spitler, A.B. Alabi, C. Foster, Z.G. Jennings, S.S. Kartha,
V. Pota
Monthly Notices of the Royal Astronomical Society, Volume 451, Issue 3, 2015 August 11, pp. 2625–2639
ADS |
MNRAS |
arXiv:1505.04795
The outer halo regions of early-type galaxies carry key information about their past
accretion history. However, spectroscopically probing the stellar component at such
galactocentric radii is still challenging. Using the DEep Imaging Multi-Object
Spectrograph on the Keck, we have been able to measure the metallicities of the
stellar and globular cluster components in 12 early-type galaxies out to more than
10 Re. We find similar metallicity gradients for the metal-poor
and metal-rich globular cluster subpopulations, suggesting a common formation
process for the two subpopulations. This is in conflict with most current
theoretical predictions, where the metal-poor globular clusters are thought to be
purely accreted and metal-rich globular clusters mostly formed in situ.
Moreover, we find that the globular cluster metallicity gradients show a trend with
galaxy mass, being steeper in lower mass galaxies than in higher mass galaxies.
This is similar to what we find for the outermost galaxy stars and suggests a more
active accretion history, with a larger role played by major mergers, in the most
massive galaxies. This conclusion is qualitatively consistent with expectations
from two-phase galaxy assembly models. Finally, we find that the small difference
in metallicity between galaxy stars and metal-rich globular clusters at 1
Re may correlate with galaxy mass. The origin of this difference
is not currently clear.
The SLUGGS survey: multipopulation dynamical modelling of the elliptical galaxy NGC 1407 from stars and globular clusters
V. Pota, A.J. Romanowsky, J.P. Brodie, J. Peñarrubia,
D.A. Forbes, N.R. Napolitano, C. Foster, M.G. Walker, J. Strader, J.C. Roediger
Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 3, 2015 July 01, pp. 3345–3358
ADS |
MNRAS |
arXiv:1504.03325
We perform in-depth dynamical modelling of the luminous and dark matter (DM)
content of the elliptical galaxy NGC 1407. Our strategy consists of solving the spherical Jeans equations for three independent dynamical tracers: stars,
blue globular clusters (GCs) and red GCs in a self-consistent manner. We adopt
a maximum-likelihood Markov Chain Monte Carlo fitting technique in the
attempt to constrain the inner slope of the DM density profile (the cusp/core
problem), and the stellar initial mass function (IMF) of the galaxy. We find
the inner logarithmic slope of the DM density profiles to be
γ = 0.6 ± 0.4, which is consistent with either a DM cusp
(γ = 1) or with a DM core (γ = 0). Our findings are
consistent with a Salpeter IMF, and marginally consistent with a Kroupa IMF.
We infer tangential orbits for the blue GCs, and radial anisotropy for red
GCs and stars. The modelling results are consistent with the virial
mass–concentration relation predicted by Λ cold dark matter (CDM)
simulations. The virial mass of NGC 1407 is log Mvir =
13.3 ± 0.2 M☉, whereas the stellar mass is
log M* = 11.8 ± 0.1 M☉. The overall
uncertainties on the mass of NGC 1407 are only 5 per cent at the projected
stellar effective radius. We attribute the disagreement between our results
and previous X-ray results to the gas not being in hydrostatic equilibrium in
the central regions of the galaxy. The halo of NGC 1407 is found be
DM-dominated, with a dynamical mass-to-light ratio of M/L =
260+174–100
M☉/L☉,B. However, this value can
be larger up to a factor of 3 depending on the assumed prior on the DM scale
radius.
Spectroscopic confirmation of the existence of large, diffuse galaxies in the Coma cluster
P.G. van Dokkum, A.J. Romanowsky, R. Abraham, J.P. Brodie, C. Conroy, M. Geha,
A. Merritt, A. Villaume, J. Zhang
The Astrophysical Journal Letters, Volume 804, Number 1, 2015 May 1, Article L26 (5pp)
ADS |
ApJL |
arXiv:1504.03320
Press coverage:
Keck |
UCO |
Yale |
Toronto |
Science/AAAS |
Daily Mail |
Gizmodo |
cnet |
boingboing |
Digital journal
We recently identified a population of low surface brightness objects in the field of the
z = 0.023 Coma cluster, using the Dragonfly Telephoto Array. Here we present Keck
spectroscopy of one of the largest of these "ultra-diffuse galaxies" (UDGs), confirming that it
is a member of the cluster. The galaxy has prominent absorption features, including the Ca II
H+K lines and the G-band, and no detected emission lines. Its radial velocity of
cz = 6280 ± 120 km s–1 is within the 1 σ velocity
dispersion of the Coma cluster. The galaxy has an effective radius of 4.3 ± 0.3 kpc and a
Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications
of tidal tails or other distortions, at least out to a radius of ~2 re.
We show that UDGs are located in a previously sparsely populated region of the
size–magnitude plane of quiescent stellar systems, as they are ~6 mag fainter than normal
early-type galaxies of the same size. It appears that the luminosity distribution of large
quiescent galaxies is not continuous, although this could largely be due to selection effects.
Dynamical measurements are needed to determine whether the dark matter halos of UDGs are
similar to those of galaxies with the same luminosity or to those of galaxies with the same size.
A SLUGGS and Gemini/GMOS combined study of the elliptical galaxy M60: wide-field photometry and kinematics of the globular cluster system
V. Pota, J.P. Brodie, T. Bridges, J. Strader, A.J. Romanowsky, A. Villaume,
Z. Jennings, F.R. Faifer, N. Pastorello, D.A. Forbes, A. Campbell, C. Usher, C. Foster,
L.R. Spiter, N. Caldwell, J.C. Forte, M.A. Norris, S.E. Zepf, M.A. Beasley,
K. Gebhardt, D.A. Hanes, R.M. Sharples, J.A. Arnold
Monthly Notices of the Royal Astronomical Society, Volume 450, Issue 2, 2015 June 21, pp. 1962–1983
ADS |
MNRAS |
arXiv:1503.07520
We present new wide-field photometry and spectroscopy of the globular
clusters (GCs) around NGC 4649 (M60), the third brightest galaxy in
the Virgo cluster. Imaging of NGC 4649 was assembled from a recently
obtained Hubble Space Telescope/Advanced Camera for Surveys mosaic,
and new Subaru/Suprime-Cam and archival
Canada–France–Hawaii Telescope/MegaCam data. About 1200
sources were followed up spectroscopically using combined observations
from three multi-object spectrographs: Keck/Deep Imaging Multi-Object
Spectrograph, Gemini/Gemini Multi-Object Spectrograph and Multiple
Mirror Telescope/Hectospec. We confirm 431 unique GCs belonging to
NGC 4649, a factor of 3.5 larger than previous data sets and with a
factor of 3 improvement in velocity precision. We confirm significant
GC colour bimodality and find that the red GCs are more centrally
concentrated, while the blue GCs are more spatially extended. We infer
negative GC colour gradients in the innermost 20 kpc and flat
gradients out to large radii. Rotation is detected along the galaxy
major axis for all tracers: blue GCs, red GCs, galaxy stars and
planetary nebulae. We compare the observed properties of NGC 4649
with galaxy formation models. We find that formation via a major
merger between two gas-poor galaxies, followed by satellite accretion,
can consistently reproduce the observations of NGC 4649 at different
radii. We find no strong evidence to support an interaction between
NGC 4649 and the neighbouring spiral galaxy NGC 4647. We identify
interesting GC kinematic features in our data, such as
counter-rotating subgroups and bumpy kinematic profiles, which encode
more clues about the formation history of NGC 4649.
Small scatter and nearly-isothermal mass profiles to four half-light radii from two-dimensional stellar dynamics of early-type galaxies
M. Cappellari, A.J. Romanowsky, J.P. Brodie, D.A. Forbes, J. Strader, C. Foster, S.S. Kartha, N. Pastorello, V. Pota, L.R. Spitler, C. Usher, J.A. Arnold
The Astrophysical Journal Letters, Volume 804, Number 1, 2015 May 1, Article L21 (7pp)
ADS |
ApJL |
arXiv:1504.00075
Press coverage:
Keck |
UCO |
Swinburne
We study the total mass-density profile for a sample of 14
fast-rotator early-type galaxies (stellar masses 10.2
≲ log M*/M☉ ≲ 11.7).
We combine observations from the SLUGGS and ATLAS3D
surveys to map out the stellar kinematics in two dimensions,
out to a median radius for the sample of four half-light radii
Re (or 10 kpc)
and a maximum radius of 2.0–6.2 Re
(or 4–21 kpc).
We use axisymmetric dynamical models based on the Jeans equations,
which allow for a spatially varying anisotropy; employ quite general
profiles for the dark halos; and, in particular, do not place any
restrictions on the profile slope. This is made possible by the
availability of spatially extended two-dimensional kinematics. We
find that our relatively simple models provide a remarkably good
description of the observed kinematics. The resulting total density
profiles are well described by a nearly isothermal power law
ρtot(r) ∝ rγ
from Re/10 to at least 4 Re, the
largest average deviation being 11%. The average logarithmic slope is
〈γ〉 = 2.19 ± 0.03 with observed rms scatter of
just σγ = 0.11.
This scatter out to large radii,
where dark matter dominates, is as small as previously reported by
lensing studies around r ≈ Re/2,
where the stars dominate. Our bulge–halo conspiracy places much
tighter constraints on galaxy formation models. It illustrates the
power of two-dimensional stellar kinematics observations at large
radii. It is now important to test the generality of our results for
different galaxy types and larger samples.
How elevated is the dynamical-to-stellar mass ratio of the ultra-compact dwarf S999?
J. Janz, D.A. Forbes, M.A. Norris, J. Strader, S.J. Penny, M. Fagioli, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 449, Issue 2, 2015 May 11, pp. 1716–1730
ADS |
MNRAS |
arXiv:1502.06598
Here we present new Keck Echelle Spectrograph and Imager high-resolution spectroscopy
and deep archival Hubble Space Telescope/Advanced Camera for Surveys imaging for
S999, an ultracompact dwarf in the vicinity of M87, which was claimed to have an
extremely high dynamical-to-stellar mass ratio. Our data increase the total integration
times by a factor of 5 and 60 for spectroscopy and imaging, respectively. This allows us
to constrain the stellar population parameters for the first time (simple stellar
population equivalent age = 7.6+2.0–1.6 Gyr; [Z/H] =
–0.95+0.12–0.10; [α/Fe] =
0.34+0.10–0.12). Assuming a Kroupa stellar initial mass
function, the stellar population parameters and luminosity
(MF814W = –12.13 ± 0.06 mag) yield a stellar mass of
M* = 3.9+0.9–0.6
×106 M☉, which we also find to be consistent
with near-infrared data. Via mass modelling, with our new measurements of velocity
dispersion (σap = 27 ± 2 km s–1) and size
(Re = 20.9 ± 1.0 pc), we obtain an elevated dynamical-to-stellar
mass ratio Mdyn/M* = 8.2 (with a range 5.6 ≤
Mdyn/M* ≤ 11.2). Furthermore, we analyse the
surface brightness profile of S999, finding only a small excess of light in the outer
parts with respect to the fitted Sérsic profile, and a positive colour gradient.
Taken together these observations suggest that S999 is the remnant of a much larger
galaxy that has been tidally stripped. If so, the observed elevated mass ratio may be
caused by mechanisms related to the stripping process: the existence of a massive
central black hole or internal kinematics that are out of equilibrium due to the
stripping event. Given the observed dynamical-to-stellar mass ratio we suggest that S999
is an ideal candidate to search for the presence of an overly massive central black hole.
VEGAS-SSS. A VST early-type galaxy survey: analysis of small stellar systems. Testing the methodology on the globular cluster system in NGC 3115
M. Cantiello, M. Capaccioli, N. Napolitano, A. Grado, L. Limatola, M. Paolillo, E. Iodice,
A.J. Romanowsky, D.A. Forbes, G. Raimondo, M. Spavone, F. La Barbera, T.H. Puzia, P. Schipani
Astronomy & Astrophysics, Volume 576, April 2015, Article A14 (19pp)
A&A |
ADS |
arXiv:1412.0469
We present a study of globular clusters (GCs) and other small stellar systems (SSSs) in the field of
NGC 3115, observed as part of the ongoing wide-field imaging survey VEGAS, carried out with the 2.6 m
VST telescope. We used deep g and i observations of NGC 3115, a well-studied lenticular galaxy that is
covered excellently well in the scientific literature. This is fundamental to test the methodologies,
verify the results, and probe the capabilities of the VEGAS-SSS. Leveraging the large field of view of
the VST allowed us to accurately study the distribution and properties of SSSs as a function of
galactocentric distance, well beyond ~20 galaxy effective radii, in a way that is rarely possible. Our
analysis of colors, magnitudes, and sizes of SSS candidates confirms the results from existing studies,
some of which were carried out with 8–10 m class telescopes, and further extends them to
previously unreached galactocentric distances with similar accuracy. In particular, we find a color
bimodality for the GC population and a de Vaucouleurs r1/4 profile for the surface density of GCs
similar to the galaxy light profile. The radial color gradient of blue and red GCs previously found,
for instance, by the SLUGGS survey with Subaru and Keck data, is further extended out to the largest
galactocentric radii inspected, ~65 kpc. In addition, the surface density profiles of blue and red GCs
taken separately are well approximated by a r1/4 density profile, with the fraction of blue GCs being
slightly larger at larger radii. We do not find hints of a trend for the red GC subpopulation and for
the GC turnover magnitude to vary with radius, but we observe a ~0.2 mag difference in the turnover
magnitude of the blue and red GC subpopulations. Finally, from inspecting SSS sizes and colors, we
obtain a list of ultracompact dwarf galaxies and GC candidates suitable for future spectroscopic
follow-up. In conclusion, our study shows i) the reliability of the methodologies developed to study
SSSs in the field of bright early-type galaxies; and ii) the great potential of the VEGAS survey to
produce original results on SSSs science, mainly thanks to the wide-field imaging adopted.