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., Labbe, I., Trenti, M., van Dokkum, P., Carollo, C. M., Gonzalez, V., and Magee, D.
Ultra-deep 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 beta, of star-forming galaxies over a range of luminosity (0.1L*(z=3) to 2L*(z=3)) at high redshift (z~7 to z~4). Beta is measured using all ACS and WFC3/IR passbands uncontaminated by Ly_alpha and spectral breaks. Extensive tests show that our beta measurements are only subject to minimal biases. Using a different selection procedure, Dunlop et al. recently found large biases in their beta measurements. To reconcile these different results, we simulated both approaches and found that beta measurements for faint sources are subject to large biases if the same passbands are used both to select the sources and to measure beta. High-redshift galaxies show a well-defined rest-frame UV color-magnitude (CM) relationship that becomes systematically bluer towards 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 (beta) plane (a "star-forming sequence"?). Dust appears to be the principal factor driving changes in the UV color (beta) with luminosity. These new larger beta samples lead to improved dust extinction estimates at z~4-7 and confirm that the extinction is still essentially zero at low luminosities and high redshifts. Inclusion of the new dust extinction results leads to (i) excellent agreement between the SFR density at z~4-8 and that inferred from the stellar mass density, and (ii) to higher SSFRs at z>~4, suggesting the SSFR may evolve modestly from z~4-7 to z~2.
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.
WFC3RED: A HST Wide Field Camera 3 Image Processing Pipeline
Magee, D. K., Bouwens, R. J., and Illingworth, G. D.
WFC3RED is a pipeline for automatically processing imaging data taken with the Wide Field Camera 3 instrument on the Hubble Space Telescope (HST). The pipeline currently supports processing of imaging data from both the IR and UVIS channels and is written in Python and C. The automated processing steps include cosmic-ray removal (UVIS), super-sky subtraction, user defined artifact masking, robust alignment and registration for large mosaics, weight map generation, and drizzling onto a final image mosaic. WFC3RED can combined data across different HST observations, visits and proposals without the need for any pre-defined associations. WFC3RED can create imaging products with a signal-to-noise ratio that matches the most careful step-by-step manual WFC3 reductions.
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.
Morphological Evolution of Galaxies from Ultra-deep Hubble Space Telescope Wide Field Camera 3 Imaging: The Hubble Sequence at z ~ 2
Szomoru, D., Franx, M., Bouwens, R. J., van Dokkum, P. G., Labbé, I., Illingworth, G. D., and Trenti, M.
We use ultra-deep Hubble Space Telescope Wide Field Camera 3/infrared imaging of the Hubble Ultra Deep Field to investigate the rest-frame optical morphologies of a mass-selected sample of galaxies at z ~ 2. We find a large variety of galaxy morphologies, ranging from large, blue, disk-like galaxies to compact, red, early-type galaxies. We derive rest-frame u - g color profiles for these galaxies and show that most z ~ 2 galaxies in our sample have negative color gradients such that their cores are red. Although these color gradients may partly be caused by radial variations in dust content, they point to the existence of older stellar populations in the centers of z ~ 2 galaxies. This result is consistent with an "inside-out" scenario of galaxy growth. We find that the median color gradient is fairly constant with redshift: (Δ(u - g rest)/Δ(log r))median = -0.47, -0.33, and -0.46 for z ~ 2, z ~ 1, and z = 0, respectively. Using structural parameters derived from surface brightness profiles we confirm that at z ~ 2 galaxy morphology correlates well with specific star formation rate. At the same mass, star-forming galaxies have larger effective radii, bluer rest-frame u - g colors, and lower Sérsic indices than quiescent galaxies. These correlations are very similar to those at lower redshift, suggesting that the relations that give rise to the Hubble sequence at z = 0 are already in place for massive galaxies at this early epoch. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
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., Labbe, 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 arcmin^2 of WFC3/IR data in the Chandra Deep Field South, using the public HUDF09, ERS, and CANDELS surveys, that reach to 5sigma 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 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. (2011). The newer data cover a 3x 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 >=4x larger than expected from the lower redshift extrapolation of the UV LF. We are thus likely witnessing the first rapid build-up of galaxies in the heart of cosmic reionization. Future deep HST WFC3/IR data, reaching to well beyond 29 mag, can enable a more robust quantification of the accelerated evolution around z~10.
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., Labbe, 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. The recent sample of 132 z~7 and z~8 galaxies from the HUDF09 data are complemented here by new large samples at z~5 and z~6 galaxies (507 and 203, respectively) and prior results at z~4. The luminosity functions we derive at these redshifts reach to very faint limits of <0.05L*(z=3). The faint-end slopes alpha 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). The results are consistent with a slope that increases systematically to earlier times, with slopes close to -2 at z~7-8. Slopes alpha ~-2 lead to formally divergent UV fluxes, though galaxies are not expected to form below ~-10 AB mag. These results have major implications for reionization. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. Adopting typical parameters and extrapolating the current LF and faint-end slope evolution from z~4-8 to z>8, we derive Thomson optical depths of 0.079, in excellent agreement with the seven-year WMAP estimates (tau=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 are large and depend critically on the trend to steeper faint-end slopes at z>=6. We show that deeper, and realistic, WFC3/IR+ACS observations can substantially lessen the uncertainties on the z~5-8 slopes and further constrain this key parameter in determining the UV ionizing flux from galaxies.
The Brightest of Reionizing Galaxies Survey: Design and Preliminary Results
Trenti, M., Bradley, L. D., Stiavelli, M., Oesch, P., Treu, T., Bouwens, R. J., Shull, J. M., MacKenty, J. W., Carollo, C. M., and Illingworth, G. D.
We present the first results on the search for very bright (M AB ≈ -21) galaxies at redshift z ~ 8 from the Brightest of Reionizing Galaxies (BoRG) survey. BoRG is a Hubble Space Telescope Wide Field Camera 3 (WFC3) pure-parallel survey that is obtaining images on random lines of sight at high Galactic latitudes in four filters (F606W, F098M, F125W, and F160W), with integration times optimized to identify galaxies at z >~ 7.5 as F098M dropouts. We discuss here results from a search area of approximately 130 arcmin2 over 23 BoRG fields, complemented by six other pure-parallel WFC3 fields with similar filters. This new search area is more than two times wider than previous WFC3 observations at z ~ 8. We identify four F098M-dropout candidates with high statistical confidence (detected at greater than 8σ confidence in F125W). These sources are among the brightest candidates currently known at z ~ 8 and approximately 10 times brighter than the z = 8.56 galaxy UDFy-38135539. They thus represent ideal targets for spectroscopic follow-up observations and could potentially lead to a redshift record, as our color selection includes objects up to z ~ 9. However, the expected contamination rate of our sample is about 30% higher than typical searches for dropout galaxies in legacy fields, such as the GOODS and HUDF, where deeper data and additional optical filters are available to reject contaminants. 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 11700, 11702.
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.
Evolution of Galaxy Stellar Mass Functions, Mass Densities, and Mass to Light Ratios from z 7 to z 4
Gonzalez, V., Labbe, I., Bouwens, R., Illingworth, G., Franx, M., and Kriek, M.
We derive stellar masses from SED fitting to rest-frame optical and UV fluxes for 401 star-forming galaxies at z 4, 5, and 6 from Hubble-WFC3/IR observations of the ERS 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/Luv 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 (MF) to masses M~10^8 at z 4-7. A bootstap approach is used to derive the MFs to account for the large scatter in the M--Luv relation and the luminosity function uncertainties, along with an analytical crosscheck. The MFs are also corrected for the effects of incompleteness. The incompleteness-corrected MFs are steeper than previously found, with slopes \alpha_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 a SMD growth \propto(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~10^{8} Msun. Such completeness-corrected MFs and the derived SMDs will be particularly important for model comparisons as future MFs reach to lower masses.
Active and passive galaxies at z~2: Rest-frame optical morphologies with WFC3
Cameron, E., Carollo, C. M., Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Trenti, M., Labbe, I., and Magee, D.
We use the high angular resolution in the near-infrared of the WFC3 on HST to determine YHVz color-color selection criteria to identify and characterize 1.5<z<3.5 galaxies in the HUDF09 and ERS (GOODS-South) fields. The WFC3 NIR images reveal galaxies at these redshifts that were undetected in the rest-frame UV HUDF/GOODS images, as well as true centers and regular disks in galaxies classified as highly irregular in rest-frame UV light. Across the 1.5<z<2.15 redshift range, regular disks are unveiled in the WFC3 images of ~25% of both intermediate and high mass galaxies, i.e., above 10^10 Msun. Meanwhile, galaxies maintaining diffuse and/or irregular morphologies in the rest-frame optical light---i.e., not yet dynamically settled---at these epochs are almost entirely restricted to masses below 10^11 Msun. In contrast at 2.25 < z < 3.5 these diffuse and/or irregular structures overwhelmingly dominate the morphological mix in both the intermediate and high mass regimes, while no regular disks, and only a small fraction (25%) of smooth spheroids, are evident above 10^11 Msun. Strikingly, by 1.5 < z < 2.25 roughly 2 out of every 3 galaxies at the highest masses are spheroids. In our small sample, the fraction of star-forming galaxies at these mass scales decreases concurrently from ~60% to ~5%. If confirmed, this indicates that z~2 is the epoch of both the morphological transformation and quenching of star-formation which assemble the first substantial population of massive ellipticals.
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.
Confirmation of the Compactness of a z = 1.91 Quiescent Galaxy with Hubble Space Telescope's Wide Field Camera 3
Szomoru, D., Franx, M., van Dokkum, P. G., Trenti, M., Illingworth, G. D., Labbé, I., Bouwens, R. J., Oesch, P. A., and Carollo, C. M.
We present very deep Wide Field Camera 3 (WFC3) photometry of a massive, compact galaxy located in the Hubble Ultra Deep Field. This quiescent galaxy has a spectroscopic redshift z = 1.91 and has been identified as an extremely compact galaxy by Daddi et al. We use new H F160W imaging data obtained with Hubble Space Telescope/WFC3 to measure the deconvolved surface brightness profile to H ≈ 28 mag arcsec-2. We find that the surface brightness profile is well approximated by an n = 3.7 Sérsic profile. Our deconvolved profile is constructed by a new technique which corrects the best-fit Sérsic profile with the residual of the fit to the observed image. This allows for galaxy profiles which deviate from a Sérsic profile. We determine the effective radius of this galaxy: re = 0.42 ± 0.14 kpc in the observed H F160W band. We show that this result is robust to deviations from the Sérsic model used in the fit. We test the sensitivity of our analysis to faint "wings" in the profile using simulated galaxy images consisting of a bright compact component and a faint extended component. We find that due to the combination of the WFC3 imaging depth and our method's sensitivity to extended faint emission we can accurately trace the intrinsic surface brightness profile, and that we can therefore confidently rule out the existence of a faint extended envelope around the observed galaxy down to our surface brightness limit. These results confirm that the galaxy lies a factor ~10 off from the local mass-size relation.
The Galaxy Luminosity Function During the Reionization Epoch
Trenti, M., Stiavelli, M., Bouwens, R. J., Oesch, P., Shull, J. M., Illingworth, G. D., Bradley, L. D., and Carollo, C. M.
The new Wide Field Camera 3/IR observations on the Hubble Ultra-Deep Field (HUDF) started investigating the properties of galaxies during the reionization epoch. To interpret these observations, we present a novel approach inspired by the conditional luminosity function method. We calibrate our model to observations at z = 6 and assume a non-evolving galaxy luminosity versus halo mass relation. We first compare model predictions against the luminosity function (LF) measured at z = 5 and z = 4. We then predict the LF at z >= 7 under the sole assumption of evolution in the underlying dark-matter halo mass function. Our model is consistent with the observed z >~ 7 galaxy number counts in the HUDF survey and suggests a possible steepening of the faint-end slope of the LF: α(z >~ 8) <~ -1.9 compared to α = -1.74 at z = 6. Although we currently see only the brightest galaxies, a hidden population of lower luminosity objects (L/L * >~ 10-4) might provide gsim75% of the total reionizing flux. Assuming escape fraction f esc ~ 0.2, clumping factor C ~ 5, top-heavy initial mass function (IMF), and low metallicity, galaxies below the detection limit produce complete reionization at z >~ 8. For solar metallicity and normal stellar IMF, reionization finishes at z >~ 6, but a smaller C/f esc is required for an optical depth consistent with the Wilkinson Microwave Anisotropy Probe measurement. Our model highlights that the star formation rate in sub-L * galaxies has a quasi-linear relation to dark-matter halo mass, suggesting that radiative and mechanical feedback were less effective at z >= 6 than today.
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.
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 λβ). 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.
Structure and Morphologies of z ~ 7-8 Galaxies from Ultra-deep WFC3/IR Imaging of the Hubble Ultra-deep Field
Oesch, P. A., Bouwens, R. J., Carollo, C. M., Illingworth, G. D., Trenti, M., Stiavelli, M., Magee, D., Labbé, I., and Franx, M.
We present a first morphological study of z ~ 7-8 Lyman break galaxies (LBGs) from Oesch et al. and Bouwens et al. detected in ultra-deep near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) by the HUDF09 program. With an average intrinsic size of 0.7 ± 0.3 kpc, these galaxies are found to be extremely compact, having an average observed surface brightness of μ J sime 26 mag arcsec-2, and only two out of the full sample of 16 z ~ 7 galaxies show extended features with resolved double cores. By comparison to lower redshift LBGs, it is found that only little size evolution takes place from z ~ 7 to z ~ 6, while galaxies between z ~ 4-5 show more extended wings in their apparent profiles. The average size scales as (1 + z)-m with m = 1.12 ± 0.17 for galaxies with luminosities in the range (0.3-1)L* z=3 and with m = 1.32 ± 0.52 for (0.12-0.3)L* z=3, consistent with galaxies having constant comoving sizes. The peak of the size distribution changes only slowly from z ~ 7 to z ~ 4. However, a tail of larger galaxies (gsim1.2 kpc) is gradually built up toward later cosmic times, possibly via hierarchical build-up or via enhanced accretion of cold gas. Additionally, the average star formation surface density of LBGs with luminosities (0.3-1)L* z=3 is nearly constant at ΣSFR = 1.9 M sun yr-1 kpc-2 over the entire redshift range z ~ 4-7 suggesting similar star formation efficiencies at these early epochs. The above evolutionary trends seem to hold out to z ~ 8 though the sample is still small and possibly incomplete. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.
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.
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.
Ultradeep Infrared Array Camera Observations of Sub-L* z ~ 7 and z ~ 8 Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity Galaxies to the Stellar Mass Density and Reionization
Labbé, I., González, V., Bouwens, R. J., Illingworth, G. D., Oesch, P. A., van Dokkum, P. G., Carollo, C. M., Franx, M., Stiavelli, M., Trenti, M., Magee, D., and Kriek, M.
We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z ~ 7 z 850-dropout galaxies and 5z ~ 8 Y 105-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6 μm and 4.5 μm. None of the galaxies are detected to [3.6] ≈ 26.9 (AB, 2σ), but a stacking analysis reveals a robust detection for the z 850-dropouts and an upper limit for the Y 105-dropouts. We construct average broadband spectral energy distributions using the stacked Advanced Camera for Surveys (ACS), WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z 850-dropouts, we find z = 6.9+0.1 -0.1, (U - V)rest ≈ 0.4, reddening AV = 0, stellar mass langM*rang = 1.2+0.3 -0.6 × 109 M sun (Salpeter initial mass function). The best-fit ages ~300 Myr, M/LV ≈ 0.2, and SSFR ~1.7 Gyr-1 are similar to values reported for luminous z ~ 7 galaxies, indicating the galaxies are smaller but not much younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7 < z < 11. In contrast, the z = 8.3+0.1 -0.2 Y 105-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2σ upper limit on the M/L(z = 8), the stellar mass density to M UV,AB < -18 declines from ρ*(z = 7) = 3.7+1.4 -1.8 × 106 M sun Mpc-3 to ρ*(z = 8) < 8 × 105 M sun Mpc-3, following vprop(1 + z)-6 over 3 < z < 8. Lower masses at z = 8 would signify more dramatic evolution, which can be established with deeper IRAC observations, long before the arrival of the James Webb Space Telescope. 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.