Latest Results

HST WFC3The Hubble UltraDeep Field 2009 (HUDF09) Project was conceived in 2007. In 2008, through a highly competitive proposal process, the HUDF09 team was awarded 192 orbits (12 days) of observations on the Hubble Space Telescope with the new Wide Field InfraRed Camera (WFC3/IR). More on the HUDF09 project...


XDF thumbnailThe Hubble eXtreme Deep Field (XDF) Project has combined 50 days of observations with the Hubble Space Telescope using the Advanced Camera for Surveys Wide Field Channel and the Wide Field Camera 3 InfraRed Channel to create the deepest image of the universe. More on the XDF project ...

Recent Publications

The Most Luminous z~9-10 Galaxy Candidates yet Found: The Luminosity Function, Cosmic Star-Formation Rate, and the First Mass Density Estimate at 500 Myr

Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Labbe, I., Smit, R., Franx, M., van Dokkum, P. G., Momcheva, I., Ashby, M. L. N., Fazio, G. G., Huang, J., Willner, S. P., Gonzalez, V., Magee, D., Brammer, G. B., and Skelton, R. E.

We present the discovery of four surprisingly bright (H_160 ~ 26 - 27 mag AB) galaxy candidates at z~9-10 in the complete HST CANDELS WFC3/IR GOODS-N imaging data, doubling the number of z~10 galaxy candidates that are known, just 500 Myr after the Big Bang. These sources were identified in a search over the full CANDELS-Deep dataset, building on our previous analysis of the HUDF09/XDF fields and GOODS-S. Three of these four galaxies are significantly detected at 4.5-6.2sigma in the very deep Spitzer/IRAC 4.5 micron data. Furthermore, the brightest of our candidates (at z=10.2+-0.4) is robustly detected also at 3.6 micron (6.9sigma), revealing a flat UV spectral energy distribution with a slope beta=-2.0+-0.2, consistent with demonstrated trends with luminosity at high redshift. The abundance of these luminous candidates suggests that the luminosity function evolves more significantly in phi_* than in L_* at z>~8. Despite the discovery of these luminous candidates, the cosmic star formation rate density for galaxies with SFR >0.7 M_sun yr^-1 shows an order-of-magnitude increase in only 170 Myr from z ~ 10 to z ~ 8, consistent with previous results. Based on the IRAC detections, we derive galaxy stellar masses at z~10, finding that these luminous objects are typically 10^9 M_sun. This allows for a first estimate of the cosmic stellar mass density at z~10 resulting in log rho* = 4.7^+0.5_-0.9 M_sun Mpc^-3 for galaxies brighter than M_UV~-18. The remarkable brightness, and hence luminosity, of these z~9-10 candidates highlights the opportunity for deep spectroscopy to determine their redshift and nature, and demonstrates the value of additional search fields to understand star-formation in the very early universe.


Probing the Dawn of Galaxies at z ~ 9-12: New Constraints from HUDF12/XDF and CANDELS data

Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Labbé, I., Franx, M., van Dokkum, P. G., Trenti, M., Stiavelli, M., Gonzalez, V., and Magee, D.

We present a comprehensive analysis of z > 8 galaxies based on ultra-deep WFC3/IR data. We exploit all the WFC3/IR imaging over the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in addition to the HUDF09 parallel field data, as well as wider area imaging over GOODS-South. Galaxies are selected based on the Lyman break technique in three samples centered around z ~ 9, z ~ 10, and z ~ 11, with seven z ~ 9 galaxy candidates, and one each at z ~ 10 and z ~ 11. We confirm a new z ~ 10 candidate (with z = 9.8 ± 0.6) that was not convincingly identified in our first z ~ 10 sample. Using these candidates, we perform one of the first estimates of the z ~ 9 UV luminosity function (LF) and improve our previous constraints at z ~ 10. Extrapolating the lower redshift UV LF evolution should have revealed 17 z ~ 9 and 9 z ~ 10 sources, i.e., a factor ~3 × and 9× larger than observed. The inferred star formation rate density (SFRD) in galaxies above 0.7 M &sun; yr-1 decreases by 0.6 ± 0.2 dex from z ~ 8 to z ~ 9, in excellent agreement with previous estimates. From a combination of all current measurements, we find a best estimate of a factor 10× decrease in the SFRD from z ~ 8 to z ~ 10, following (1 + z)-11.4 ± 3.1. Our measurements thus confirm our previous finding of an accelerated evolution beyond z ~ 8, and signify a very rapid build-up of galaxies with M UV < -17.7 mag within only ~200 Myr from z ~ 10 to z ~ 8, in the heart of cosmic reionization. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc., for NASA under contract NAS5-26555.


A Rest-frame Optical View on z ~ 4 Galaxies. I. Color and Age Distributions from Deep IRAC Photometry of the IUDF10 and GOODS Surveys

Oesch, P. A., Labbé, I., Bouwens, R. J., Illingworth, G. D., Gonzalez, V., Franx, M., Trenti, M., Holden, B. P., van Dokkum, P. G., and Magee, D.

We present a study of rest-frame UV-to-optical color distributions for z ~ 4 galaxies based on the combination of deep HST/ACS+WFC3/IR data with Spitzer/IRAC imaging. In particular, we use new, ultra-deep data from the IRAC Ultradeep Field program (IUDF10), together with previous, public IRAC data over the GOODS fields. Our sample contains a total of ~2600 galaxies selected as B-dropout Lyman-break Galaxies in the HUDF and its deep parallel field HUDF09-2, as well as GOODS-North/South. This sample is used to investigate the UV continuum slopes β and Balmer break colors (J 125 - [4.5]) as a function of rest-frame optical luminosity (using [4.5] to avoid optical emission lines). We find that galaxies at Mz < -21.5 (roughly corresponding to L^*_{z\sim 4}) are significantly redder than their lower luminosity counterparts. The UV continuum slopes and the J 125 - [4.5] colors are well correlated, indicating that the dust reddening at these redshifts is better described by an SMC-like extinction curve, rather than the typically assumed Calzetti reddening. After dust correction, we find that the galaxy population shows mean stellar population ages in the range 108.5 to 109 yr, with a dispersion of ~0.5 dex, and only weak trends as a function of luminosity. Only a small fraction of galaxies shows Balmer break colors consistent with extremely young ages, younger than 100 Myr. Under the assumption of smooth star-formation histories, this fraction is 12%-19% for galaxies at Mz < -19.75. Our results are consistent with a gradual build-up of stars and dust in galaxies at z > 4 with only a small fraction of stars being formed in short, intense bursts of star-formation. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555. Based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.


UV-Continuum Slopes of >4000 z~4-8 Galaxies from the HUDF/XDF, HUDF09, ERS, CANDELS-South, and CANDELS-North Fields

Bouwens, R. J., Illingworth, G. D., Oesch, P. A., Labbe, I., van Dokkum, P. G., Trenti, M., Franx, M., Smit, R., Gonzalez, V., and Magee, D.

We measure the UV-continuum slope beta for over 4000 high-redshift galaxies over a wide range of redshifts z~4-8 and luminosities from the HST HUDF/XDF, HUDF09-1, HUDF09-2, ERS, CANDELS-N, and CANDELS-S data sets. Our new beta results reach very faint levels at z~4 (-15.5 mag: 0.006 L*(z=3)), z~5 (-16.5 mag: 0.014L*(z=3)), and z~6 and z~7 (-17 mag: 0.025 L*(z=3)). Inconsistencies between previous studies led us to conduct a comprehensive review of systematic errors and develop a new technique for measuring beta that is robust against biases that arise from the impact of noise. We demonstrate, by object-by-object comparisons, that all previous studies, including our own and those done on the latest HUDF12 dataset, suffer from small systematic errors in beta. We find that after correcting for the systematic errors (typically d(beta) ~0.1-0.2) all beta results at z~7 from different groups are in excellent agreement. The mean beta we measure for faint (-18 mag: 0.1L*(z=3)) z~4, z~5, z~6, and z~7 galaxies is -2.03+/-0.03+/-0.06 (random and systematic errors), -2.14+/-0.06+/-0.06, -2.24+/-0.11+/-0.08, and -2.33+/-0.16+/-0.13, respectively. Our new beta values are redder than we have reported in the past, but bluer than other recent results. Our previously reported trend of bluer beta's at lower luminosities is confirmed, as is the evolution to bluer beta's at high redshifts. beta appears to show only a mild luminosity dependence faintward of M(UV,AB) ~ -19 mag, suggesting that the mean beta asymptotes to ~ -2.2 to -2.4 for faint z>~4 galaxies. At z~7, the observed beta's suggest non-zero, but low dust extinction, and they agree well with values predicted in cosmological hydrodynamical simulations.


The HST eXtreme Deep Field XDF: Combining all ACS and WFC3/IR Data on the HUDF Region into the Deepest Field Ever

Illingworth, G. D., Magee, D., Oesch, P. A., Bouwens, R. J., Labbe, I., Stiavelli, M., van Dokkum, P. G., Franx, M., Trenti, M., Carollo, C. M., and Gonzalez, V.

The eXtreme Deep Field (XDF) combines data from ten years of observations with the HST Advanced Camera for Surveys (ACS) and the Wide-Field Camera 3 Infra-Red (WFC3/IR) into the deepest image of the sky ever in the optical/near-IR. Since the initial observations on the Hubble Ultra-Deep Field (HUDF) in 2003, numerous surveys and programs, including supernova followup, HUDF09, CANDELS, and HUDF12 have contributed additional imaging data across the HUDF region. Yet these have never been combined and made available as one complete ultra-deep optical and near-infrared image dataset. We do so now for the eXtreme Deep Field (XDF) program. Our new and improved processing techniques provide higher quality reductions of the total dataset. All WFC3 near-IR and optical ACS data sets have been fully combined and accurately matched, resulting in the deepest imaging ever taken at these wavelengths ranging from 29.1 to 30.3 AB mag (5sigma in a 0.35" diameter aperture) in 9 filters. The gains in the optical for the four filters done in the original ACS HUDF correspond to a typical improvement of 0.15 mag, with gains of 0.25 mag in the deepest areas. Such gains are equivalent to adding ~130 to ~240 orbits of ACS data to the HUDF. Improved processing alone results in a typical gain of ~0.1 mag. Our 5sigma (optical+near-IR) SExtractor catalogs reveal about 14140 sources in the full field and about 7121 galaxies in the deepest part of the XDF (the HUDF09 region). The XDF is the deepest image of the universe ever taken, reaching, in the combined image for a flat f_nu source, to 31.2 AB mag 5sigma (32.9 at 1sigma) in a 0.35" diameter aperture.


Photometric Constraints on the Redshift of z ~ 10 Candidate UDFj-39546284 from Deeper WFC3/IR+ACS+IRAC Observations over the HUDF

Bouwens, R. J., Oesch, P. A., Illingworth, G. D., Labbé, I., van Dokkum, P. G., Brammer, G., Magee, D., Spitler, L. R., Franx, M., Smit, R., Trenti, M., Gonzalez, V., and Carollo, C. M.

Ultra-deep WFC3/IR observations on the HUDF from the HUDF09 program revealed just one plausible z ~ 10 candidate, UDFj-39546284. UDFj-39546284 had all the properties expected of a galaxy at z ~ 10 showing (1) no detection in the deep ACS+WFC3 imaging data blueward of the F160W band, exhibiting (2) a blue spectral slope redward of the break, and showing (3) no prominent detection in deep IRAC observations. The new, similarly deep WFC3/IR HUDF12 F160W observations over the HUDF09/XDF allow us to further assess this candidate. These observations show that this candidate, previously only detected at ~5.9σ in a single band, clearly corresponds to a real source. It is detected at ~5.3σ in the new H 160-band data and at ~7.8σ in the full 85-orbit H 160-band stack. Interestingly, the non-detection of the source (<1σ) in the new F140W observations suggests a higher redshift. Formally, the best-fit redshift of the source utilizing all the WFC3+ACS (and IRAC+Ks -band) observations is 11.8 ± 0.3. However, we consider the z ~ 12 interpretation somewhat unlikely, since the source would either need to be ~20× more luminous than expected or show very high-EW Lyα emission (which seems improbable given the extensive neutral gas prevalent early in the reionization epoch). Lower-redshift solutions fail if only continuum models are allowed. Plausible lower-redshift solutions require that the H 160-band flux be dominated by line emission such as Hα or [O III] with extreme EWs. The tentative detection of line emission at 1.6 μm in UDFj-39546284 in a companion paper suggests that such emission may have already been found. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.


A Tentative Detection of an Emission Line at 1.6 μm for the z ~ 12 Candidate UDFj-39546284

Brammer, G. B., van Dokkum, P. G., Illingworth, G. D., Bouwens, R. J., Labbé, I., Franx, M., Momcheva, I., and Oesch, P. A.

We present deep WFC3 grism observations of the candidate z ~ 12 galaxy UDFj-39546284 in the Hubble Space Telescope (HST) Ultra Deep Field (UDF), by combining spectroscopic data from the 3D-HST and CANDELS surveys. The total exposure time is 40.5 ks and the spectrum covers 1.10 < λ < 1.65 μm. We search for faint emission lines by cross-correlating the two-dimensional G141 spectrum with the observed H 160 morphology, a technique that is unique to slitless spectroscopy at HST resolution. We find a 2.7σ detection of an emission line at 1.599 μm—just redward of the JH 140 filter—with flux 3.5 ± 1.3 × 10-18 erg s-1 cm-2. Assuming that the line is real, it contributes 110% ± 40% of the observed H 160 flux and has an observed equivalent width >7300 Å. If the line is confirmed, it could be Lyα at z = 12.12. However, a more plausible interpretation, given current results, could be a lower redshift feature such as [O III]λ4959,5007 at z = 2.19. We find two other 3D-HST [O III] emitters within 1000 km s-1 of that redshift in the GOODS-South field. Additional support for this interpretation comes from the discovery of a bright "[O III] blob" with a secure G141 grism redshift of z = 1.605. This object has a strikingly large observed equivalent width of nearly 9000 Å that results in similar "dropout" colors as UDFj-39546284. Based on observations made with the NASA/ESA Hubble Space Telescope, programs GO-12099, 12177, and 12547, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.


The Bright End of the Ultraviolet Luminosity Function at z ~ 8: New Constraints from CANDELS Data in GOODS-South

Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Gonzalez, V., Trenti, M., van Dokkum, P. G., Franx, M., Labbé, I., Carollo, C. M., and Magee, D.

We present new z ~ 8 galaxy candidates from a search over ~95 arcmin2 of WFC3/IR data, tripling the previous search area for bright z ~ 8 galaxies. Our analysis uses newly acquired WFC3/IR imaging data from the CANDELS Multi-Cycle Treasury program over the GOODS-South field. These new data are combined with existing deep optical Advanced Camera for Surveys (ACS) imaging to search for relatively bright (M UV < -19.5 mag) z ~ 8 galaxy candidates using the Lyman break technique. These new candidates are used to determine the bright end of the UV luminosity function (LF) of star-forming galaxies at z ~ 7.2-8.7, i.e., a cosmic age of 600 ± 80 Myr. To minimize contamination from lower redshift galaxies, we make full use of all optical ACS data and impose strict non-detection criteria based on an optical χ2 opt flux measurement. In the whole search area, we identify 16 candidate z ~ 8 galaxies, spanning a magnitude range H 160, AB = 25.7-27.9 mag. The new data show that the UV LF is a factor ~1.7 lower at M UV < -19.5 mag than determined from the HUDF09 and Early Release Science (ERS) data alone. Combining this new sample with the previous candidates from the HUDF09 and ERS data allows us to perform the most accurate measurement of the z ~ 8 UV LF yet. Schechter function fits to the combined data result in a best-fit characteristic magnitude of M *(z = 8) = -20.04 ± 0.46 mag. The faint-end slope is very steep, though quite uncertain, with α = -2.06 ± 0.32. A combination of wide-area data with additional ultra-deep imaging will be required to significantly reduce the uncertainties on these parameters in the future. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.


The Spectral Energy Distributions Of z~8 Galaxies From The IRAC Ultra Deep Fields: Emission Lines, Stellar Masses, And Specific Star Formation Rates At 650 Myr

Labbe, I., Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Magee, D., Gonzalez, V., Carollo, C. M., Franx, M., Trenti, M., van Dokkum, P. G., and Stiavelli, M.

Using new ultradeep Spitzer/IRAC photometry from the IRAC Ultradeep Field program (IUDF), we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z~8, only 650Myr after the Big Bang. The sources are selected from HST/ACS+WFC3/IR data over the Hubble Ultra Deep Field (HUDF), two HUDF parallel fields, and wide area data over the CANDELS/GOODS-South. The new Spitzer/IRAC data increase the coverage at 3.6 micron and 4.5 micron to ~120h over the HUDF reaching depths of ~28 (AB,1 sigma). The improved depth and inclusion of brighter candidates result in direct >3 sigma IRAC detections of 20/63 sources, of which 11/63 are detected at > 5 sigma. The average [3.6]-[4.5] colors of IRAC detected galaxies at z~8 are markedly redder than those at z~7, observed only 130Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [OIII]4959,5007+Hbeta) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z~7 and z~8 we estimate a rest-frame equivalent width of W([OIII]4959,5007+Hbeta) = 670 (+260,-170) Angstrom contributing 0.56 (+0.16,-0.11) mag to the 4.5 micron filter at z~8. The corresponding W(Halpha) = 430 (+160,-110) Angstrom implies an average specific star formation rate of sSFR = 11 (+11,-5) Gyr^-1 and a stellar population age of 100 (+100,-50) Myr. Correcting the spectral energy distribution for the contribution of emission lines lowers the average best-fit stellar masses and mass-to-light ratios by x3, decreasing the integrated stellar mass density to rho*(z=8,MUV<-18)=0.6 (+0.4,-0.3) x 10^6 Msun Mpc^-3.


The Star Formation Rate Function for Redshift z ~ 4-7 Galaxies: Evidence for a Uniform Buildup of Star-forming Galaxies during the First 3 Gyr of Cosmic Time

Smit, R., Bouwens, R. J., Franx, M., Illingworth, G. D., Labbé, I., Oesch, P. A., and van Dokkum, P. G.

We combine recent estimates of dust extinction at z ~ 4-7 with UV luminosity function (LF) determinations to derive star formation rate (SFR) functions at z ~ 4-7. SFR functions provide a more physical description of galaxy buildup at high redshift and allow for direct comparisons to SFRs at lower redshifts determined by a variety of techniques. Our SFR functions are derived from well-established z ~ 4-7 UV LFs, UV-continuum slope trends with redshift and luminosity, and infrared excess (IRX)-β relations. They are well described by Schechter relations. We extend the comparison baseline for SFR functions to z ~ 2 by considering recent determinations of the Hα and mid-IR LFs. The low-end slopes of the SFR functions are flatter than for the UV LFs, Δα ~ +0.13, and show no clear evolution with cosmic time (z ~ 0-7). In addition, we find that the characteristic value SFR* from the Schechter fit to the SFR function exhibits consistent, and substantial, linear growth as a function of redshift from ~5 M &sun; yr-1 at z ~ 8, 650 Myr after the big bang, to ~100 M &sun; yr-1 at z ~ 2, ~2.5 Gyr later. Recent results at z ~ 10, close to the onset of galaxy formation, are consistent with this trend. The uniformity of this evolution is even greater than seen in the UV LF over the redshift range z ~ 2-8, providing validation for our dust corrections. These results provide strong evidence that galaxies build up uniformly over the first 3 Gyr of cosmic time.


UV-continuum Slopes at z ~ 4-7 from the HUDF09+ERS+CANDELS Observations: Discovery of a Well-defined UV Color-Magnitude Relationship for z >= 4 Star-forming Galaxies

Bouwens, R. J., Illingworth, G. D., Oesch, P. A., Franx, M., Labbé, I., Trenti, M., van Dokkum, P., Carollo, C. M., González, V., Smit, R., and Magee, D.

Ultra-deep Advanced Camera for Surveys (ACS) and WFC3/IR HUDF+HUDF09 data, along with the wide-area GOODS+ERS+CANDELS data over the CDF-S GOODS field, are used to measure UV colors, expressed as the UV-continuum slope β, of star-forming galaxies over a wide range of luminosity (0.1L* z = 3 to 2L* z = 3) at high redshift (z ~ 7 to z ~ 4). β is measured using all ACS and WFC3/IR passbands uncontaminated by Lyα and spectral breaks. Extensive tests show that our β measurements are only subject to minimal biases. Using a different selection procedure, Dunlop et al. recently found large biases in their β measurements. To reconcile these different results, we simulated both approaches and found that β measurements for faint sources are subject to large biases if the same passbands are used both to select the sources and to measure β. High-redshift galaxies show a well-defined rest-frame UV color-magnitude (CM) relationship that becomes systematically bluer toward fainter UV luminosities. No evolution is seen in the slope of the UV CM relationship in the first 1.5 Gyr, though there is a small evolution in the zero point to redder colors from z ~ 7 to z ~ 4. This suggests that galaxies are evolving along a well-defined sequence in the L UV-color (β) plane (a "star-forming sequence"?). Dust appears to be the principal factor driving changes in the UV color β with luminosity. These new larger β samples lead to improved dust extinction estimates at z ~ 4-7 and confirm that the extinction is essentially zero at low luminosities and high redshifts. Inclusion of the new dust extinction results leads to (1) excellent agreement between the star formation rate (SFR) density at z ~ 4-8 and that inferred from the stellar mass density; and (2) to higher specific star formation rates (SSFRs) at z >~ 4, suggesting that the SSFR may evolve modestly (by factors of ~2) from z ~ 4-7 to z ~ 2. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 11563 and 9797.


Lower-luminosity Galaxies Could Reionize the Universe: Very Steep Faint-end Slopes to the UV Luminosity Functions at z >= 5-8 from the HUDF09 WFC3/IR Observations

Bouwens, R. J., Illingworth, G. D., Oesch, P. A., Trenti, M., Labbé, I., Franx, M., Stiavelli, M., Carollo, C. M., van Dokkum, P., and Magee, D.

The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LFs) from these new HUDF09 data for 132 z ~ 7 and z ~ 8 galaxies are combined with new LFs for z ~ 5-6 galaxies and the earlier z ~ 4 LF to reach to very faint limits (<0.05 L* z = 3). The faint-end slopes α are steep: -1.79 ± 0.12 (z ~ 5), -1.73 ± 0.20 (z ~ 6), -2.01 ± 0.21 (z ~ 7), and -1.91 ± 0.32 (z ~ 8). Slopes α <~ -2 lead to formally divergent UV fluxes, though galaxies are not expected to form below ~ - 10 AB mag. These results have important implications for reionization. The weighted mean slope at z ~ 6-8 is -1.87 ± 0.13. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. While current results show that galaxies can reionize the universe by z ~ 6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z > 8, taking α to be -1.87 ± 0.13 (the mean value at z ~ 6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061+0.009 - 0.006. However, this result will change if the faint-end slope α is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (dα/dz ~ -0.05 ± 0.04) that would increase the Thomson optical depths to 0.079+0.063 - 0.017, consistent with recent WMAP estimates (τ = 0.088 ± 0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the UV ionizing flux from faint galaxies. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.


Through the Looking Glass: Bright, Highly Magnified Galaxy Candidates at z ~ 7 behind A1703

Bradley, L. D., Bouwens, R. J., Zitrin, A., Smit, R., Coe, D., Ford, H. C., Zheng, W., Illingworth, G. D., Benítez, N., and Broadhurst, T. J.

We report the discovery of seven strongly lensed Lyman-break galaxy (LBG) candidates at z ~ 7 detected in Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging of A1703. The brightest candidate, called A1703-zD1, has an observed (lensed) magnitude of 24.0 AB (26σ) in the WFC3/IR F160W band, making it 0.2 mag brighter than the z 850-dropout candidate recently reported behind the Bullet Cluster and 0.7 mag brighter than the previously brightest known z ~ 7.6 galaxy, A1689-zD1. With a cluster magnification of ~9, this source has an intrinsic magnitude of H 160 = 26.4 AB, a strong z 850 - J 125 break of 1.7 mag, and a photometric redshift of z ~ 6.7. Additionally, we find six other bright LBG candidates with H 160-band magnitudes of 24.9-26.4, photometric redshifts z ~ 6.4 - 8.8, and magnifications μ ~ 3-40. Stellar population fits to the Advanced Camera for Surveys, WFC3/IR, and Spitzer/Infrared Array Camera data for A1703-zD1 and A1703-zD4 yield stellar masses (0.7 - 3.0) × 109 M &sun;, stellar ages 5-180 Myr, and star formation rates ~7.8 M &sun; yr-1, and low reddening with AV <= 0.7. The source-plane reconstruction of the exceptionally bright candidate A1703-zD1 exhibits an extended structure, spanning ~4 kpc in the z ~ 6.7 source plane, and shows three resolved star-forming knots of radius r ~ 0.4 kpc. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS5-26555. Based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.


Expanded Search for z ~ 10 Galaxies from HUDF09, ERS, and CANDELS Data: Evidence for Accelerated Evolution at z > 8?

Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Labbé, I., Trenti, M., Gonzalez, V., Carollo, C. M., Franx, M., van Dokkum, P. G., and Magee, D.

We search for z ~ 10 galaxies over ~160 arcmin2 of Wide-Field Camera 3 (WFC3)/IR data in the Chandra Deep Field South, using the public HUDF09, Early Release Science, and CANDELS surveys, that reach to 5σ depths ranging from 26.9 to 29.4 in H 160 AB mag. z >~ 9.5 galaxy candidates are identified via J 125 - H 160 > 1.2 colors and non-detections in any band blueward of J 125. Spitzer Infrared Array Camera (IRAC) photometry is key for separating the genuine high-z candidates from intermediate-redshift (z ~ 2-4) galaxies with evolved or heavily dust obscured stellar populations. After removing 16 sources of intermediate brightness (H 160 ~ 24-26 mag) with strong IRAC detections, we only find one plausible z ~ 10 galaxy candidate in the whole data set, previously reported in Bouwens et al.. The newer data cover a 3 × larger area and provide much stronger constraints on the evolution of the UV luminosity function (LF). If the evolution of the z ~ 4-8 LFs is extrapolated to z ~ 10, six z ~ 10 galaxies are expected in our data. The detection of only one source suggests that the UV LF evolves at an accelerated rate before z ~ 8. The luminosity density is found to increase by more than an order of magnitude in only 170 Myr from z ~ 10 to z ~ 8. This increase is >=4 × larger than expected from the lower redshift extrapolation of the UV LF. We are thus likely witnessing the first rapid buildup of galaxies in the heart of cosmic reionization. Future deep Hubble Space Telescope WFC3/IR data, reaching to well beyond 29 mag, can enable a more robust quantification of the accelerated evolution around z ~ 10. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc., for NASA under contract NAS5-26555. Partially based on observations made with the Spitzer Space Telescope, operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.


Ultraviolet Luminosity Functions from 132 z ~ 7 and z ~ 8 Lyman-break Galaxies in the Ultra-deep HUDF09 and Wide-area Early Release Science WFC3/IR Observations

Bouwens, R. J., Illingworth, G. D., Oesch, P. A., Labbé, I., Trenti, M., van Dokkum, P., Franx, M., Stiavelli, M., Carollo, C. M., Magee, D., and Gonzalez, V.

We identify 73 z ~ 7 and 59 z ~ 8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to < - 18 AB mag and the star formation rate (SFR) density at z ~ 7 and z ~ 8 (just 800 Myr and 650 Myr after recombination, respectively). The galaxy sample is derived using a sophisticated Lyman-break technique on the full two-year Wide Field Camera 3/infrared (WFC3/IR) and Advanced Camera for Surveys (ACS) data available over the HUDF09 (~29.4 AB mag, 5σ), two nearby HUDF09 fields (~29 AB mag, 5σ, 14 arcmin2), and the wider area Early Release Science (~27.5 AB mag, 5σ, ~40 arcmin2). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just ~7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, active galactic nuclei, spurious sources, low-mass stars, and transients (e.g., supernovae). From careful modeling of the selection volumes for each of our search fields, we derive luminosity functions for galaxies at z ~ 7 and z ~ 8 to < - 18 AB mag. The faint-end slopes α at z ~ 7 and z ~ 8 are uncertain but very steep at α = -2.01 ± 0.21 and α = -1.91 ± 0.32, respectively. Such steep slopes contrast to the local α >~ -1.4 and may even be steeper than that at z ~ 4 where α = -1.73 ± 0.05. With such steep slopes (α <~ -1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The SFR densities derived from these new z ~ 7 and z ~ 8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency with the SFR densities implied from reported stellar mass densities being only ~40% higher at z < 7. This suggests that (1) the stellar mass densities inferred from the Spitzer Infrared Array Camera (IRAC) photometry are reasonably accurate and (2) that the initial mass function at very high redshift may not be very different from that at later times. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 11563, 9797, and 10632.


Evolution of Galaxy Stellar Mass Functions, Mass Densities, and Mass-to-light Ratios from z ~ 7 to z ~ 4

González, V., Labbé, I., Bouwens, R. J., Illingworth, G., Franx, M., and Kriek, M.

We derive stellar masses from spectral energy distribution fitting to rest-frame optical and UV fluxes for 401 star-forming galaxies at z ~ 4, 5, and 6 from Hubble-WFC3/IR camera observations of the Early Release Science field combined with the deep GOODS-S Spitzer/IRAC data (and include a previously published z ~ 7 sample). A mass-luminosity relation with strongly luminosity-dependent {M}/L_UV ratios is found for the largest sample (299 galaxies) at z ~ 4. The relation {M}\propto L_{UV,1500}^{1.7({+/- }0.2)} has a well-determined intrinsic sample variance of 0.5 dex. This relation is also consistent with the more limited samples at z ~ 5-7. This z ~ 4 mass-luminosity relation, and the well-established faint UV-luminosity functions at z ~ 4-7, are used to derive galaxy mass functions (MFs) to masses {M}\sim 10^{8} at z ~ 4-7. A bootstrap approach is used to derive the MFs to account for the large scatter in the {M}{--}L_UV relation and the luminosity function uncertainties, along with an analytical cross-check. The MFs are also corrected for the effects of incompleteness. The incompleteness-corrected MFs are steeper than previously found, with slopes α M ~ -1.4 to -1.6 at low masses. These slopes are, however, still substantially flatter than the MFs obtained from recent hydrodynamical simulations. We use these MFs to estimate the stellar mass density (SMD) of the universe to a fixed M UV, AB < - 18 as a function of redshift and find an SMD growth vprop(1 + z)-3.4 ± 0.8 from z ~ 7 to z ~ 4. We also derive the SMD from the completeness-corrected MFs to a mass limit {M}\sim 10^{8} M sun. Such completeness-corrected MFs and the derived SMDs will be particularly important for comparisons as future MFs reach to lower masses.


A candidate redshift z~10 galaxy and rapid changes in that population at an age of 500Myr

Bouwens, R. J., Illingworth, G. D., Labbe, I., Oesch, P. A., Trenti, M., Carollo, C. M., van Dokkum, P. G., Franx, M., Stiavelli, M., González, V., Magee, D., and Bradley, L.

Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900-2,000million years (Myr) after the Big Bang (redshifts 6>z>3 ref. 1). The Hubble Ultra Deep Field (HUDF09) data have yielded the first reliable detections of z~8 galaxies that, together with reports of a γ-ray burst at z~8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z~7-8 galaxies suggest substantial star formation at z>9-10 (refs 12, 13). Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z~10 galaxies in the heart of the reionization epoch, only 500Myr after the Big Bang. Not only do we find one possible z~10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (~10%) at this time than it is just ~200Myr later at z~8. This demonstrates how rapid galaxy build-up was at z~10, as galaxies increased in both luminosity density and volume density from z~10 to z~8. The 100-200Myr before z~10 is clearly a crucial phase in the assembly of the earliest galaxies.


The Evolution of the Ultraviolet Luminosity Function from z ~ 0.75 to z ~ 2.5 Using HST ERS WFC3/UVIS Observations

Oesch, P. A., Bouwens, R. J., Carollo, C. M., Illingworth, G. D., Magee, D., Trenti, M., Stiavelli, M., Franx, M., Labbé, I., and van Dokkum, P. G.

We present UV luminosity functions (LFs) at 1500 Å derived from the Hubble Space Telescope Early Release Science WFC3/UVIS data acquired over ~50 arcmin2 of the GOODS-South field. The LFs are determined over the entire redshift range z = 0.75-2.5 using two methods, similar to those used at higher redshifts for Lyman break galaxies (LBGs): (1) 13 band UV+optical+NIR photometric redshifts to study galaxies in the range z = 0.5-2 in three bins of dz = 0.5 and (2) dropout samples in three redshift windows centered at z ~ 1.5, z ~ 1.9, and z ~ 2.5. The characteristic luminosity dims by 1.5 mag from z = 2.5 to z = 0.75, consistent with earlier work. However, the other Schechter function parameters, the faint-end slope and the number density, are found to be remarkably constant over the range z = 0.75-2.5. Using these LF determinations, we find the UV luminosity density to increase by ~1.4 dex according to (1 + z)2.58±0.15 from z ~ 0 to its peak at z ~ 2.5. Strikingly, the inferred faint-end slopes for our LFs are all steeper than α = -1.5, in agreement with higher-redshift LBG studies. Since the faint-end slope in the local universe is found to be much flatter with α ~= -1.2, this poses the question as to when and how the expected flattening occurs. Despite relatively large uncertainties, our data suggest α ~= -1.7 at least down to z ~ 1. These new results from such a shallow early data set demonstrate very clearly the remarkable potential of WFC3/UVIS for the thorough characterization of galaxy evolution over the full redshift range z ~ 0.5 to z ~ 3. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc., for NASA under contract NAS5-26555.


z ~ 7 Galaxy Candidates from NICMOS Observations Over the HDF-South and the CDF-South and HDF-North Goods Fields

Bouwens, R. J., Illingworth, G. D., González, V., Labbé, I., Franx, M., Conselice, C. J., Blakeslee, J., van Dokkum, P., Holden, B., Magee, D., Marchesini, D., and Zheng, W.

We use ~88 arcmin2 of deep (gsim26.5 mag at 5σ) NICMOS data over the two GOODS fields and the HDF-South to conduct a search for bright z >~ 7 galaxy candidates. This search takes advantage of an efficient preselection over 58 arcmin2 of NICMOS H 160-band data where only plausible z >~ 7 candidates are followed up with NICMOS J 110-band observations. ~248 arcmin2 of deep ground-based near-infrared data (gsim25.5 mag, 5σ) are also considered in the search. In total, we report 15 z 850-dropout candidates over this area—7 of which are new to these search fields. Two possible z ~ 9 J 110-dropout candidates are also found, but seem unlikely to correspond to z ~ 9 galaxies (given the estimated contamination levels). The present z ~ 9 search is used to set upper limits on the prevalence of such sources. Rigorous testing is undertaken to establish the level of contamination of our selections by photometric scatter, low-mass stars, supernovae, and spurious sources. The estimated contamination rate of our z ~ 7 selection is ~24%. Through careful simulations, the effective volume available to our z >~ 7 selections is estimated and used to establish constraints on the volume density of luminous (L* z = 3, or ~-21 mag) galaxies from these searches. We find that the volume density of luminous star-forming galaxies at z ~ 7 is 13+8 -5 times lower than at z ~ 4 and >25 times lower (1σ) at z ~ 9 than at z ~ 4. This is the most stringent constraint yet available on the volume density of gsimL* z = 3 galaxies at z ~ 9. The present wide-area, multi-field search limits cosmic variance to lsim20%. The evolution we find at the bright end of the UV LF is similar to that found from recent Subaru Suprime-Cam, HAWK-I or ERS WFC3/IR searches. The present paper also includes a complete summary of our final z ~ 7 z 850-dropout sample (18 candidates) identified from all NICMOS observations to date (over the two GOODS fields, the HUDF, galaxy clusters). Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #7235, 7817, 9425, 9575, 9723, 9797, 9803, 9978, 9979, 10189, 10339, 10340, 10403, 10530, 10632, 10872, 11082, 11144, and 11192. Observations have been carried out using the Very Large Telescope at the ESO Paranal Observatory under Program ID(s): LP168.A-0485.


Star Formation Rates and Stellar Masses of z = 7-8 Galaxies from IRAC Observations of the WFC3/IR Early Release Science and the HUDF Fields

Labbé, I., González, V., Bouwens, R. J., Illingworth, G. D., Franx, M., Trenti, M., Oesch, P. A., van Dokkum, P. G., Stiavelli, M., Carollo, C. M., Kriek, M., and Magee, D.

We investigate the Spitzer/IRAC properties of 36 z ~ 7 z 850-dropout galaxies and three z ~ 8 Y 098 galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the spectral energy distributions to derive mean redshifts, stellar masses, and ages. The z ~ 7 galaxies are best characterized by substantial ages (>100 Myr) and M/LV ≈ 0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from β ~ -2.0 to β ~ -3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1L* z = 3), where low metallicity and smooth star formation histories (SFHs) fail to match the blue far-UV and moderately red H - [3.6] color. Such colors may require episodic SFHs with short periods of activity and quiescence ("on-off" cycles) and/or a contribution from emission lines. The stellar mass of our sample of z ~ 7 star-forming galaxies correlates with star formation rate (SFR) according to log M* = 8.70(±0.09) + 1.06(±0.10)log SFR, implying that star formation may have commenced at z > 10. No galaxies are found with SFRs much higher or lower than the past averaged SFR suggesting that the typical star formation timescales are probably a substantial fraction of the Hubble time. We report the first IRAC detection of Y 098-dropout galaxies at z ~ 8. The average rest-frame U - V ≈ 0.3 (AB) of the three galaxies are similar to faint z ~ 7 galaxies, implying similar M/L. The stellar mass density to M UV,AB < -18 is ρ*(z = 8) = 1.8+0.7 -1.0 × 106 M sun Mpc-3, following log ρ*(z) = 10.6(±0.6) - 4.4(±0.7) log(1 + z) [M sun Mpc-3] over 3 < z < 8. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 11563, 9797. Based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through contract 125790 issued by JPL/Caltech. Based on service mode observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 073.A-0764A). Based on data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.