R. J. Bouwens

UV Luminosity Functions at z~4, 5, and 6 from the Hubble Ultra Deep Field and Other Deep Hubble Space Telescope ACS Fields: Evolution and Star Formation History*

We use the ACS BViz data from the HUDF and all other deep HST ACS fields (including the GOODS fields) to find large samples of star-forming galaxies at z ~ 4 and ~5 and to extend our previous z ~ 6 sample. These samples contain 4671, 1416, and 627 B-, V-, and i-dropouts, respectively, and reach to extremely low luminosities [(0.01-0.04)L or MUV ~ -16 to -17], allowing us to determine the rest-frame UV LF and faint-end slope ? at z ~ 4-6 to high accuracy. We find faint-end slopes ? = -1.73 ? 0.05, -1.66 ? 0.09, and -1.74 ? 0.16 at z ~ 4, ~5, and ~6, respectively, suggesting that the faint-end slope is very steep and shows little evolution with cosmic time. We find that M brightens considerably in the 0.7 Gyr from z ~ 6 to ~4 (by ~0.7 mag from M = -20.24 ? 0.19 to -20.98 ? 0.10). The observed increase in the characteristic luminosity over this range is almost identical to that expected for the halo mass function, suggesting that the observed evolution is likely due to the hierarchical coalescence and merging of galaxies. The evolution in * is not significant. The UV luminosity density at z ~ 6 is modestly lower than (0.45 ? 0.09 times) that at z ~ 4 (integrated to -17.5 mag) although a larger change is seen in the dust-corrected SFR density. We thoroughly examine published LF results and assess the reasons for their wide dispersion. We argue that the results reported here are the most robust available. The extremely steep faint-end slopes ? found here suggest that lower luminosity galaxies play a significant role in reionizing the universe. Finally, recent search results for galaxies at z ~ 7-8 are used to extend our estimates of the evolution of M* from z ~ 7-8 to z ~ 4.

UV Luminosity Functions from 113 z~7 and z~8 Lyman-Break Galaxies in the ultra-deep HUDF09 and wide-area ERS WFC3/IR Observations

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 density at z~7 and z~8. The galaxy sample is derived using a sophisticated Lyman-Break technique on the full two-year WFC3/IR and ACS data available over the HUDF09 (~29.4 AB mag, 5 sigma), two nearby HUDF09 fields (~29 AB mag, 14 arcmin) and the wider area ERS (~27.5 AB mag) ~40 arcmin**2). 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, AGN, spurious sources, low mass stars, and transients (e.g., SNe). From careful modelling 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 alpha at z~7 and z~8 are uncertain but very steep at alpha = -2.01+/-0.21 and alpha=-1.91+/-0.32, respectively. Such steep slopes contrast to the local alpha<~-1.4 and may even be steeper than that at z~4 where alpha=-1.73+/-0.05. With such steep slopes (alpha<~-1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The star formation rate 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 IRAC photometry are reasonably accurate and (2) that the IMF at very high redshift may not be very different from that at later times.

UV LUMINOSITY FUNCTIONS AT REDSHIFTS z ∼ 4 TO z ∼ 10: 10,000 GALAXIES FROM HST LEGACY FIELDS* **

The remarkable Hubble Space Telescope?(HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) from to . We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at , , , , , and , respectively, from the ?1000 arcmin2 area covered by these data sets. This sample of >10,000 galaxy candidates at is by far the largest assembled to date with HST. The selection of 4?8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at . Our new LF determinations at and span a 6 mag baseline and reach to ?16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to significance for a steeper faint-end slope of the UV LF at , with ? evolving from at to at (and at ), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M* from to the observed evolution in the LF is now largely represented by changes in . No evidence for a non-Schechter-like form to the z ? 4?8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio of halos provides a good representation of the observed evolution.

Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z ~ 2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse

Using deep near-infrared spectroscopy, Kriek et al. found that ∼45% of massive galaxies at have evolved z ∼ 2.3 stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)–assisted Keck/adaptive optics (AO). Considering that their median stellar mass is , the galaxies are remarkably small, with 11 1.7 # 10 M, a median effective radius kpc. Galaxies of similar mass in the nearby universe have sizes of ≈5 kpc and r p 0.9 e average stellar densities that are 2 orders of magnitude lower than the galaxies. These results extend earlier z ∼ 2.3 work at and confirm previous studies at that lacked spectroscopic redshifts and imaging of sufficient z ∼ 1.5 z 1 2 resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most ∼10% of the population of K-selected quiescent galaxies at , effectively ruling out simple monolithic z ∼ 2.3 models for their formation. The galaxies must evolve significantly after , through dry mergers or other z ∼ 2.3 processes, consistent with predictions from hierarchical models. Subject headings: cosmology: observations — galaxies: evolution — galaxies: formation

The Morphology-Density Relation in z ~ 1 Clusters

We measure the morphology-density relation (MDR) and morphology-radius relation (MRR) for galaxies in seven z ~ 1 clusters that have been observed with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope. Simulations and independent comparisons of our visually derived morphologies indicate that ACS allows one to distinguish between E, S0, and spiral morphologies down to z850 = 24, corresponding to L/L* = 0.21 and 0.30 at z = 0.83 and 1.24, respectively. We adopt density and radius estimation methods that match those used at lower redshift in order to study the evolution of the MDR and MRR. We detect a change in the MDR between 0.8 < z < 1.2 and that observed at z ~ 0, consistent with recent work; specifically, the growth in the bulge-dominated galaxy fraction, fE+S0, with increasing density proceeds less rapidly at z ~ 1 than it does at z ~ 0. At z ~ 1 and Σ ≥ 500 galaxies Mpc-2, we find fE+S0 = 0.72 ± 0.10. At z ~ 0, an E+S0 population fraction of this magnitude occurs at densities about 5 times smaller. The evolution in the MDR is confined to densities Σ 40 galaxies Mpc-2 and appears to be primarily due to a deficit of S0 galaxies and an excess of Sp+Irr galaxies relative to the local galaxy population. The fE-density relation exhibits no significant evolution between z = 1 and 0. We find mild evidence to suggest that the MDR is dependent on the bolometric X-ray luminosity of the intracluster medium. Implications for the evolution of the disk galaxy population in dense regions are discussed in the context of these observations.

Galaxies at z ~ 6: The UV Luminosity Function and Luminosity Density from 506 HUDF, HUDF Parallel ACS Field, and GOODS i-Dropouts* **

We have detected 506 i-dropouts (z ~ 6 galaxies) in deep, wide-area HST ACS fields: HUDF, enhanced GOODS, and HUDF parallel ACS fields (HUDF-Ps). The contamination levels are 8% (i.e., 92% are at z ~ 6). With these samples, we present the most comprehensive, quantitative analyses of z ~ 6 galaxies yet and provide optimal measures of the UV luminosity function (LF) and luminosity density at z ~ 6, and their evolution to z ~ 3. We redetermine the size and color evolution from z ~ 6 to z ~ 3. Field-to-field variations (cosmic variance), completeness, flux, and contamination corrections are modeled systematically and quantitatively. After corrections, we derive a rest-frame continuum UV (~1350 ?) LF at z ~ 6 that extends to M1350,AB ~ -17.5 (0.04L). There is strong evidence for evolution of the LF between z ~ 6 and z ~ 3, most likely through a brightening (0.6 ? 0.2 mag) of M* (at 99.7% confidence), although the degree depends on the faint-end slope. As expected from hierarchical models, the most luminous galaxies are deficient at z ~ 6. Density evolution (*) is ruled out at >99.99% confidence. Despite large changes in the LF, the luminosity density at z ~ 6 is similar to (0.82 ? 0.21 times) that at z ~ 3. Changes in the mean UV color of galaxies from z ~ 6 to z ~ 3 suggest an evolution in dust content, indicating that the true evolution is substantially larger: at z ~ 6 the star formation rate density is just ~30% of the z ~ 3 value. Our UV LF is consistent with z ~ 6 galaxies providing the necessary UV flux to reionize the universe.

z ~ 7-10 Galaxies in the HUDF and GOODS Fields: UV Luminosity Functions

We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at -->z 7 and constrain the UV LF within the first 700 Myr. Our data set includes ~23 arcmin2 of deep NICMOS -->J + H data and ~248 arcmin2 of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find eight --> ~ 7.3 z-dropouts in our search fields, but no -->z ~ 9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ~12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at -->z ~ 7 and -->z ~ 9. For a faint-end slope -->α = − 1.74, our most likely values for -->MUV* and * at -->z ~ 7 are – -->19.8 ± 0.4 mag and -->1.1+ 1.7−0.7 × 10−3 Mpc−3, respectively. Our search results for -->z ~ 9 J-dropouts set a 1 σ lower limit on -->MUV* of –19.6 mag assuming that * and α are the same as their values at slightly later times. This lower limit on -->MUV* is 1.4 mag fainter than our best-fit value at -->z ~ 4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from -->z ~ 7 to -->z ~ 6 and at 80% confidence from -->z ~ 9 to -->z ~ 7. We find that the mass-to-light ratio of halos evolves as ~ -->(1 + z)−1 if we require that the observed brightening in -->MUV* with redshift [i.e., -->MUV* = (− 21.02 ± 0.09) + (0.36 ± 0.08) (z − 3.8) ] be consistent with the expected evolution in the halo mass function. Finally, we consider the shape of the UV LF at -->z 5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cutoff at the bright end.

Size Evolution of the Most Massive Galaxies at 1.7 < z < 3 from GOODS NICMOS Survey Imaging

We measure the sizes of 82 massive (M ≥ 1011 M☉) galaxies at 1.7 ≤ z ≤ 3 utilizing deep HST NICMOS data taken in the GOODS North and South fields. Our sample is almost an order of magnitude larger than previous studies at these redshifts, providing the first statistical study of massive galaxy sizes at z > 2, confirming the extreme compactness of these systems. We split our sample into disk-like (n ≤ 2) and spheroid-like (n > 2) galaxies based on their Sersic indices, and find that at a given stellar mass disk-like galaxies at z ~ 2.3 are a factor of 2.6 ± 0.3 smaller than present-day equal-mass systems, and spheroid-like galaxies at the same redshifts are 4.3 ± 0.7 smaller than comparatively massive elliptical galaxies today. At z > 2 our results are compatible with both a leveling off, or a mild evolution in size. Furthermore, the high density (~2 × 1010 M☉ kpc−3) of massive galaxies at these redshifts, which are similar to present-day globular clusters, possibly makes any further evolution in sizes beyond z = 3 unlikely.

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

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??m and 4.5??m 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 z = 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 ? 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 ? yr-1. From this sample, we measure an SMD of 6.6+5.4 ?3.3 ? 105 M ? Mpc-3 to a limit of M UV,AB 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.

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

Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900–2,000 million 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 500 Myr 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 ∼200 Myr 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 100–200 Myr before z ≈ 10 is clearly a crucial phase in the assembly of the earliest galaxies.