Recent Publications

Constraints on the First Galaxies: z~10 Galaxy Candidates from HST WFC3/IR

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

The first galaxies likely formed a few hundred million years after the Big Bang. Until recently, it has not been possible to detect galaxies earlier than ~750 million years after the Big Bang. The new HST WFC3/IR camera changed this when the deepest-ever, near-IR image of the universe was obtained with the HUDF09 program. Here we use this image to identify three redshift z~10 galaxy candidates in the heart of the reionization epoch when the universe was just 500 million years old. These would be the highest redshift galaxies yet detected, higher than the recent detection of a GRB at z~8.2. The HUDF09 data previously revealed galaxies at z~7 and z~8. Galaxy stellar population models predict substantial star formation at z>9-10. Verification by direct observation of the existence of galaxies at z~10 is the next step. Our conservative search and extensive testing for contamination and spurious images suggests that we can set reliable constraints based upon our 3 z~10 candidates, unlike a recent claim of 20 z~10 sources which appear to be spurious. The detection of galaxies at z>8 is further enhanced by our detailed analysis of 2 other faint sources likely at z~8.4 and z~8.7. Our z~10 sample suggests that the luminosity function and star formation rate density evolution found at lower redshifts continues to z~10, and pushes back the timescale for early galaxy buildup to z>10, increasing the likely role of galaxies in providing the UV flux needed to reionize the universe. The true nature of these galaxies, at just 4% of the age of the universe, will remain hidden until JWST is launched.

2009arXiv0912.4263B

Discovery of z ~ 8 Galaxies in the Hubble Ultra Deep Field from Ultra-Deep WFC3/IR Observations

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

We utilize the newly acquired, ultra-deep WFC3/IR observations over the Hubble Ultra Deep Field (HUDF) to search for star-forming galaxies at z ~ 8-8.5, only 600 million years from recombination, using a Y ₁₀₅-dropout selection. The new 4.7 arcmin² WFC3/IR observations reach to ~28.8 AB mag (5σ) in the Y ₁₀₅ J ₁₂₅ H ₁₆₀ bands. These remarkable data reach ~1.5 AB mag deeper than the previous data over the HUDF, and now are an excellent match to the HUDF optical ACS data. For our search criteria, we use a two-color Lyman break selection technique to identify z ~ 8-8.5Y ₁₀₅-dropouts. We find five likely z ~ 8-8.5 candidates. The sources have H ₁₆₀-band magnitudes of ~28.3 AB mag and very blue UV-continuum slopes, with a median estimated β of lsim-2.5 (where f _λ vprop λ^β). This suggests that z ~ 8 galaxies are not only essentially dust free but also may have very young ages or low metallicities. The observed number of Y ₁₀₅-dropout candidates is smaller than the 20 ± 6 sources expected assuming no evolution from z ~ 6, but is consistent with the five expected extrapolating the Bouwens et al. luminosity function (LF) results to z ~ 8. These results provide evidence that the evolution in the LF seen from z ~ 7 to z ~ 3 continues to z ~ 8. The remarkable improvement in the sensitivity of WFC3/IR has enabled Hubble Space Telescope to cross a threshold, revealing star-forming galaxies at z~ 8-9. 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.

2010ApJ...709L.133B

z ~ 7 Galaxies in the HUDF: First Epoch WFC3/IR Results

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

We present a sample of 16 robust z ~ 7 z ₈₅₀-drop galaxies detected by the newly installed Wide Field Camera 3 (WFC3)/IR on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y ₁₀₅, J ₁₂₅, and H ₁₆₀ observations. These remarkable data cover 4.7 arcmin² and are the deepest near infrared images ever taken, reaching to ~29 mag AB (5σ). The 16 z ~ 6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z ₈₅₀ - Y ₁₀₅) versus (Y ₁₀₅ - J ₁₂₅) colors. They have magnitudes J ₁₂₅=26.0-29.0 (AB), an average apparent half-light radius of ~0.16 arcsec (lsim1 kpc), and show very blue colors (some even β lsim -2.5), in particular at low luminosities. The WFC3/IR data confirm previous Near Infrared Camera and Multi-Object Spectrometer detections indicating that the dropout selection at z ~ 7 is very reliable. Our data allow a first determination of the faint-end slope of the z ~ 7 luminosity function, reaching down to M _UV ~ -18, a full magnitude fainter than previous measurements. When fixing phi_* = 1.4 × 10^-3 Mpc^-3 mag^-1 to the value previously measured at z ~ 6, we find a best-fit value of α = -1.77 ± 0.20, with a characteristic luminosity of M _* = -19.91 ± 0.09. This steep slope is similar to what is seen at z ~ 2-6 and indicates that low-luminosity galaxies could potentially provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of the James Webb Space Telescope. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.

2010ApJ...709L..16O

Very Blue UV-Continuum Slope β of Low Luminosity z ~ 7 Galaxies from WFC3/IR: Evidence for Extremely Low Metallicities?

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

We use the ultra-deep WFC3/IR data over the Hubble Ultra Deep Field and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z ~ 7. We determine the UV-continuum slope β in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in β. For luminous L* _z=3 galaxies, we measure a mean UV-continuum slope β of -2.0 ± 0.2, which is comparable to the β ~ -2 derived at similar luminosities at z ~ 5-6. However, for the lower luminosity 0.1L* _z=3 galaxies, we measure a mean β of -3.0 ± 0.2. This is substantially bluer than is found for similar luminosity galaxies at z ~ 4, just 800 Myr later, and even at z ~ 5-6. In principle, the observed β of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected β becomes >=-2.7. To produce these very blue βs (i.e., β ~ -3), extremely low metallicities and mechanisms to reduce the red nebular emission seem to be required. For example, a large escape fraction (i.e., f _esc gsim 0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z ~ 7 galaxies is gsim0.3, it may help to explain how galaxies reionize the universe. 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.

2010ApJ...708L..69B

The Stellar Mass Density and Specific Star Formation Rate of the Universe at z ~ 7

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

We use a robust sample of 11 z ~ 7 galaxies (z ₈₅₀ dropouts) to estimate the stellar mass density (SMD) of the universe when it was only ~750 Myr old. We combine the very deep optical to near-infrared photometry from the Hubble Space Telescope Advanced Camera for Surveys and NICMOS cameras with mid-infrared Spitzer Infrared Array Camera (IRAC) imaging available through the GOODS program. After carefully removing the flux from contaminating foreground sources, we have obtained reliable photometry in the 3.6 μm and 4.5 μm IRAC channels. The spectral shapes of these sources, including their rest-frame optical colors, strongly support their being at z ~ 7 with a mean photometric redshift of langzrang = 7.2 ± 0.5. We use Bruzual & Charlot synthetic stellar population models to constrain their stellar masses and star formation histories. We find stellar masses that range over (0.1-12) × 10⁹ M _sun and average ages from 20 Myr to 425 Myr with a mean of ~300 Myr, suggesting that in some of these galaxies most of the stars were formed at z > 8 (and probably at z >~ 10). The best fits to the observed SEDs are consistent with little or no dust extinction, in agreement with recent results at z ~ 4-8. The star formation rates (SFRs) are in the range from 5 to 20 M _sun yr^-1. From this sample, we measure an SMD of 6.6^+5.4 _-3.3 × 10⁵ M _sun Mpc^-3 to a limit of M _UV,AB < -20 (or 0.4L* _{z = 3}). Combined with a fiducial lower limit for their ages (80 Myr), this implies a maximum SFR density of 0.008 M _sun yr^-1 Mpc^-3. This is well below the critical level needed to reionize the universe at z ~ 8 using standard assumptions. However, this result is based on luminous sources (>L*) and does not include the dominant contribution of the fainter galaxies. Strikingly, we find that the specific SFR is constant from z ~ 7 to z ~ 2 but drops substantially at more recent times.

2010ApJ...713..115G