# XDFPROJECT

#### The XDF Paper

The XDF team has published a paper titled "The HST eXtreme Deep Field XDF: Combining all ACS and WFC3/IR Data on the HUDF Region into the Deepest Field Ever" discussing the methods used to produce the XDF dataset.

#### The XDF Data Release v1.0

The XDF team has release the XDF v1.0 data products on the MAST HST data archive. These data include all optical and infrared data taken by Hubble covering the original Hubble Ultra-Deep Field (HUDF) program.

The data can be downloaded from the MAST High Level Science Products page XDF Data Release 1.0

## XDF Data Paper

### 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., Labbé, 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 10 years of observations with the Hubble Space Telescope 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 of the Hubble Ultra-Deep Field (HUDF) in 2003, numerous surveys and programs, including supernovae follow-up, HUDF09, CANDELS, and HUDF12, have contributed additional imaging data across this region. However, these images have never been combined and made available as one complete ultra-deep image dataset. We combine them now with the XDF program. Our new and improved processing techniques provide higher quality reductions of the total dataset. All WFC3/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 (5σ in a 0.''35 diameter aperture) in 9 filters. The combined image therefore reaches to 31.2 AB mag 5σ (32.9 at 1σ) for a flat f ν source. 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 5σ (optical+near-IR) SExtractor catalogs reveal about 14,140 sources in the full field and about 7121 galaxies in the deepest part of the XDF.

## Papers Using the XDF

### High-precision Photometric Redshifts from Spitzer/IRAC: Extreme [3.6]-[4.5] Colors Identify Galaxies in the Redshift Range z~6.6-6.9

Smit, R., Bouwens, R. J., Franx, M., Oesch, P. A., Ashby, M. L. N., Willner, S. P., Labbe, I., Holwerda, B., Fazio, G. G., and Huang, J.-S.

One of the most challenging aspects of studying galaxies in the z>~7 universe is the infrequent confirmation of their redshifts through spectroscopy, a phenomenon thought to occur from the increasing opacity of the intergalactic medium to Lya photons at z>6.5. The resulting redshift uncertainties inhibit the efficient search for [C II] in z~7 galaxies with sub-mm instruments such as ALMA, given their limited scan speed for faint lines. One means by which to improve the precision of the inferred redshifts is to exploit the potential impact of strong nebular emission lines on the colors of z~4-8 galaxies as observed by Spitzer/IRAC. At z~6.8, galaxies exhibit IRAC colors as blue as [3.6]-[4.5] ~-1, likely due to the contribution of [O III]+Hb to the 3.6 mum flux combined with the absence of line contamination in the 4.5 mum band. In this paper we explore the use of extremely blue [3.6]-[4.5] colors to identify galaxies in the narrow redshift window z~6.6-6.9. When combined with an I-dropout criterion, we demonstrate that we can plausibly select a relatively clean sample of z~6.8 galaxies. Through a systematic application of this selection technique to our catalogs from all five CANDELS fields, we identify 20 probable z~6.6-6.9 galaxies. We estimate that our criteria select the ~50% strongest line emitters at z~6.8 and from the IRAC colors we estimate a typical [O III]+Hb rest-frame equivalent width of 1085A for this sample. The small redshift uncertainties on our sample make it particularly well suited for follow-up studies with facilities such as ALMA.

### Lens Models and Magnification Maps of the Six Hubble Frontier Fields Clusters

Johnson, T. L., Sharon, K., Bayliss, M. B., Gladders, M. D., Coe, D., and Ebeling, H.

We present strong-lensing models as well as mass and magnification maps for the cores of the six Hubble Space Telescope (HST) Frontier Fields galaxy clusters. Our parametric lens models are constrained by the locations and redshifts of multiple image systems of lensed background galaxies. We use a combination of photometric redshifts and spectroscopic redshifts of the lensed background sources obtained by us (for A2744 and AS1063), collected from the literature, or kindly provided by the lensing community. Using our results, we (1) compare the derived mass distribution of each cluster to its light distribution, (2) quantify the cumulative magnification power of the HST Frontier Fields clusters, (3) describe how our models can be used to estimate the magnification and image multiplicity of lensed background sources at all redshifts and at any position within the cluster cores, and (4) discuss systematic effects and caveats resulting from our modeling methods. We specifically investigate the effect of the use of spectroscopic and photometric redshift constraints on the uncertainties of the resulting models. We find that the photometric redshift estimates of lensed galaxies are generally in excellent agreement with spectroscopic redshifts, where available. However, the flexibility associated with relaxed redshift priors may cause the complexity of large-scale structure that is needed to account for the lensing signal to be underestimated. Our findings thus underline the importance of spectroscopic arc redshifts, or tight photometric redshift constraints, for high precision lens models. All products from our best-fit lens models (magnification, convergence, shear, deflection field) and model simulations for estimating errors are made available via the Mikulski Archive for Space Telescopes. Based on observations made with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile and the NASA/ESA Hubble Space Telescope, obtained through the Mikulski Archive for Space Telescopes (MAST) 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. These observations are associated with program 13495.

### Accounting for Cosmic Variance in Studies of Gravitationally Lensed High-redshift Galaxies in the Hubble Frontier Field Clusters

Robertson, B. E., Ellis, R. S., Dunlop, J. S., McLure, R. J., Stark, D. P., and McLeod, D.

Strong gravitational lensing provides a powerful means for studying faint galaxies in the distant universe. By magnifying the apparent brightness of background sources, massive clusters enable the detection of galaxies fainter than the usual sensitivity limit for blank fields. However, this gain in effective sensitivity comes at the cost of a reduced survey volume and, in this Letter, we demonstrate that there is an associated increase in the cosmic variance uncertainty. As an example, we show that the cosmic variance uncertainty of the high-redshift population viewed through the Hubble Space Telescope Frontier Field cluster Abell 2744 increases from ~35% at redshift z ~ 7 to >~ 65% at z ~ 10. Previous studies of high-redshift galaxies identified in the Frontier Fields have underestimated the cosmic variance uncertainty that will affect the ultimate constraints on both the faint-end slope of the high-redshift luminosity function and the cosmic star formation rate density, key goals of the Frontier Field program.

### Introducing the Illustris project: the evolution of galaxy populations across cosmic time

Genel, S., Vogelsberger, M., Springel, V., Sijacki, D., Nelson, D., Snyder, G., Rodriguez-Gomez, V., Torrey, P., and Hernquist, L.

We present an overview of galaxy evolution across cosmic time in the Illustris simulation. Illustris is an N-body/hydrodynamical simulation that evolves 2 × 18203 resolution elements in a (106.5 Mpc)3 box from cosmological initial conditions down to z = 0 using the AREPO moving-mesh code. The simulation uses a state-of-the-art set of physical models for galaxy formation that was tuned to reproduce the z = 0 stellar mass function and the history of the cosmic star formation rate density. We find that Illustris successfully reproduces a plethora of observations of galaxy populations at various redshifts, for which no tuning was performed, and provide predictions for future observations. In particular, we discuss (a) the buildup of galactic mass, showing stellar mass functions and the relations between stellar mass and halo mass from z = 7 to 0, (b) galaxy number density profiles around massive central galaxies out to z = 4, (c) the gas and total baryon content of both galaxies and their haloes for different redshifts, and as a function of mass and radius, and (d) the evolution of galaxy specific star formation rates up to z = 8. In addition, we (i) present a qualitative analysis of galaxy morphologies from z = 5 to 0, for the stellar as well as the gaseous components, and their appearance in Hubble Space Telescope mock observations, (ii) follow galaxies selected at z = 2 to their z = 0 descendants, and quantify their growth and merger histories, and (iii) track massive z = 0 galaxies to high redshift and study their joint evolution in star formation activity and compactness. We conclude with a discussion of several disagreements with observations, and lay out possible directions for future research.

### Young Galaxy Candidates in the Hubble Frontier Fields. I. A2744

Zheng, W., Shu, X., Moustakas, J., Zitrin, A., Ford, H. C., Huang, X., Broadhurst, T., Molino, A., Diego, J. M., Infante, L., Bauer, F. E., Kelson, D. D., and Smit, R.

We report the discovery of 24 Lyman-break candidates at 7 <~ z <~ 10.5, in the Hubble Frontier Fields (HFF) imaging data of A2744 (z = 0.308), plus Spitzer/IRAC data and archival ACS data. The sample includes a triple image system with a photometric redshift of z ~= 7.4. This high redshift is geometrically confirmed by our lens model corresponding to deflection angles that are 12% larger than the lower-redshift systems used to calibrate the lens model at z = 2.019. The majority of our high-redshift candidates are not expected to be multiply lensed given their locations in the image plane and the brightness of foreground galaxies, but are magnified by factors of ~1.3-15, so that we are seeing further down the luminosity function than comparable deep-field imaging. It is apparent that the redshift distribution of these sources does not smoothly extend over the full redshift range accessible at z < 12, but appears to break above z = 9. Nine candidates are clustered within a small region of 20'' across, representing a potentially unprecedented concentration. Given the poor statistics, however, we must await similar constraints from the additional HFF clusters to properly examine this trend. The physical properties of our candidates are examined using the range of lens models developed for the HFF program by various groups including our own, for a better estimate of underlying systematics. Our spectral-energy-distribution fits for the brightest objects suggest stellar masses of ~= 109 M &sun;, star formation rates of ~= 4 M &sun; yr–1, and a typical formation redshift of z <~ 19.

### A physical model for the redshift evolution of high-z Lyman-break galaxies

Jose, C., Srianand, R., and Subramanian, K.

We present a galaxy formation model to understand the evolution of stellar mass (M*)-UV luminosity relations, stellar mass functions and specific star formation rate (sSFR) of Lyman-break galaxies (LBGs) along with their UV luminosity functions (LFs) in the redshift range 3 ≤ z ≤ 8. Our models assume a physically motivated form for star formation in galaxies and model parameters are calibrated by fitting the observed UV LFs of LBGs. We find the fraction of baryons that gets converted into stars remains nearly constant for z ≥ 4 but shows an increase for z < 4. However, the rate of converting baryons into stars does not evolve significantly in the redshift range 3 ≤ z ≤ 8. Our model further successfully explains the M*-UV luminosity (MAB) correlations of LBGs. While our model predictions of stellar mass functions compare well with the inferred data from observations at the low-mass end, we need to invoke the Eddington bias to fit the high-mass end. At any given redshift, we find the sSFR to be constant over the stellar mass range 5 × 108-5 × 109 M&sun; and the redshift evolution of sSFR is well approximated by a form (1 + z)2.4 for 3 ≤ z ≤ 8 which is consistent with observations. Thus, we find that dark matter halo buildup in the ΛCDM model is sufficient to explain the evolution of UV LFs of LBGs along with their M*-MAB relations, the stellar mass functions and the sSFR for 3 ≤ z ≤ 8.

### 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., Labbé, I., van Dokkum, P. G., Trenti, M., Franx, M., Smit, R., Gonzalez, V., and Magee, D.

We measure the UV-continuum slope β 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 β results reach very faint levels at z ~ 4 (–15.5 mag: 0.006 Lz=3*), z ~ 5 (–16.5 mag: 0.014 Lz=3*), and z ~ 6 and z ~ 7 (–17 mag: 0.025 Lz=3*). Inconsistencies between previous studies led us to conduct a comprehensive review of systematic errors and develop a new technique for measuring β 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 data set, suffered from small systematic errors in β. We find that after correcting for the systematic errors (typically Δβ ~ 0.1-0.2) all β results at z ~ 7 from different groups are in excellent agreement. The mean β we measure for faint (–18 mag: 0.1 Lz=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 –2.30 ± 0.18 ± 0.13, respectively. Our new β values are redder than we have reported in the past, but bluer than other recent results. Our previously reported trend of bluer β's at lower luminosities is confirmed, as is the evolution to bluer β's at high redshifts. β appears to show only a mild luminosity dependence faintward of M UV, AB ~ –19 mag, suggesting that the mean β asymptotes to ~–2.2 to –2.4 for faint z >= 4 galaxies. At z ~ 7, the observed β's suggest non-zero, but low dust extinction, and they agree well with values predicted in cosmological hydrodynamical simulations.

### The Hawk-I UDS and GOODS Survey (HUGS): Survey design and deep K-band number counts

Fontana, A., Dunlop, J. S., Paris, D., Targett, T. A., Boutsia, K., Castellano, M., Galametz, A., Grazian, A., McLure, R., Merlin, E., Pentericci, L., Wuyts, S., Almaini, O., Caputi, K., Chary, R.-R., Cirasuolo, M., Conselice, C. J., Cooray, A., Daddi, E., Dickinson, M., Faber, S. M., Fazio, G., Ferguson, H. C., Giallongo, E., Giavalisco, M., Grogin, N. A., Hathi, N., Koekemoer, A. M., Koo, D. C., Lucas, R. A., Nonino, M., Rix, H. W., Renzini, A., Rosario, D., Santini, P., Scarlata, C., Sommariva, V., Stark, D. P., van der Wel, A., Vanzella, E., Wild, V., Yan, H., and Zibetti, S.

We present the results of a new, ultra-deep, near-infrared imaging survey executed with the Hawk-I imager at the ESO VLT, of which we make all the data (images and catalog) public. This survey, named HUGS (Hawk-I UDS and GOODS Survey), provides deep, high-quality imaging in the K and Y bands over the portions of the UKIDSS UDS and GOODS-South fields covered by the CANDELS HST WFC3/IR survey. In this paper we describe the survey strategy, the observational campaign, the data reduction process, and the data quality. We show that, thanks to exquisite image quality and extremely long exposure times, HUGS delivers the deepest K-band images ever collected over areas of cosmological interest, and in general ideally complements the CANDELS data set in terms of image quality and depth. In the GOODS-S field, the K-band observations cover the whole CANDELS area with a complex geometry made of 6 different, partly overlapping pointings, in order to best match the deep and wide areas of CANDELS imaging. In the deepest region (which includes most of the Hubble Ultra Deep Field) exposure times exceed 80 hours of integration, yielding a 1 - σ magnitude limit per square arcsec of ≃28.0 AB mag. The seeing is exceptional and homogeneous across the various pointings, confined to the range 0.38-0.43 arcsec. In the UDS field the survey is about one magnitude shallower (to match the correspondingly shallower depth of the CANDELS images) but includes also Y-band band imaging (which, in the UDS, was not provided by the CANDELS WFC3/IR imaging). In the K-band, with an average exposure time of 13 hours, and seeing in the range 0.37-0.43 arcsec, the 1 - σ limit per square arcsec in the UDS imaging is ≃27.3 AB mag. In the Y-band, with an average exposure time ≃8 h, and seeing in the range 0.45-0.5 arcsec, the imaging yields a 1 - σ limit per square arcsec of ≃28.3 AB mag. We show that the HUGS observations are well matched to the depth of the CANDELS WFC3/IR data, since the majority of even the faintest galaxies detected in the CANDELS H-band images are also detected in HUGS. Finally we present the K-band galaxy number counts produced by combining the HUGS data from the two fields. We show that the slope of the number counts depends sensitively on the assumed distribution of galaxy sizes, with potential impact on the estimated extra-galactic background light. All the HUGS images and catalogues are made publicly available at the ASTRODEEP website (http://www.astrodeep.eu) as well as from the ESO archive.Full Table 3 is available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A11

### First Frontier Field Constraints on the Cosmic Star-Formation Rate Density at z~10 - The Impact of Lensing Shear on Completeness of High-Redshift Galaxy Samples

Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Franx, M., Ammons, S. M., van Dokkum, P. G., Trenti, M., and Labbe, I.

We search the complete Hubble Frontier Field dataset of Abell 2744 and its parallel field for z~10 sources to further refine the evolution of the cosmic star-formation rate density (SFRD) at z>8. We independently confirm two images of the recently discovered triply-imaged z~9.8 source by Zitrin et al. (2014) and set an upper limit for similar z~10 galaxies with red colors of J_125-H_160>1.2 in the parallel field of Abell 2744. We utilize extensive simulations to derive the effective selection volume of Lyman-break galaxies at z~10, both in the lensed cluster field and in the adjacent parallel field. Particular care is taken to include position-dependent lensing shear to accurately account for the expected sizes and morphologies of highly-magnified sources. We show that both source blending and shear reduce the completeness at a given observed magnitude in the cluster, particularly near the critical curves. These effects have a significant, but largely overlooked, impact on the detectability of high-redshift sources behind clusters, and substantially reduce the expected number of highly-magnified sources. The detections and limits from both pointings result in a SFRD which is higher by 0.4+-0.4 dex than previous estimates at z~10 from blank fields. Nevertheless, the combination of these new results with all other estimates remain consistent with a rapidly declining SFRD in the 170 Myr from z~8 to z~10 as predicted by cosmological simulations and dark-matter halo evolution in LambdaCDM. Once biases introduced by magnification dependent completeness are accounted for, the full six cluster and parallel Frontier Field program will be an extremely powerful new dataset to probe the evolution of the galaxy population at z>8 before the advent of the JWST.

### Measurement of Galaxy Clustering at z ~ 7.2 and the Evolution of Galaxy Bias from 3.8 < z < 8 in the XDF, GOODS-S, and GOODS-N

Barone-Nugent, R. L., Trenti, M., Wyithe, J. S. B., Bouwens, R. J., Oesch, P. A., Illingworth, G. D., Carollo, C. M., Su, J., Stiavelli, M., Labbe, I., and van Dokkum, P. G.

Lyman-break galaxy (LBG) samples observed during reionization (z >~ 6) with the Hubble Space Telescope's Wide Field Camera 3 are reaching sizes sufficient to characterize their clustering properties. Using a combined catalog from the Hubble eXtreme Deep Field and CANDELS surveys, containing N = 743 LBG candidates at z >= 6.5 at a mean redshift of \overline{z}=7.2, we detect a clear clustering signal in the angular correlation function (ACF) at >~ 4σ, corresponding to a real-space correlation length r_0=6.7^{+0.9}_{-1.0}h^{-1}cMpc. The derived galaxy bias b=8.6^{+0.9}_{-1.0} is that of dark matter halos of M=10^{11.1^{+0.2}_{-0.3}} M_{\odot }\hspace{2.84526pt}at z = 7.2, and highlights that galaxies below the current detection limit (MAB ~ -17.7) are expected in lower-mass halos (M ~ 108-1010.5 M_{\odot }\hspace{2.84526pt}). We compute the ACF of LBGs at z ~ 3.8 - z ~ 5.9 in the same surveys. A trend of increasing bias is found from \overline{z}=3.8 (b ~ 3.0) to \overline{z}=7.2 (b ~ 8.6), broadly consistent with galaxies at fixed luminosity being hosted in dark matter halos of similar mass at 4 <~ z <~ 6, followed by a slight rise in halo masses at z >~ 7 (~2σ confidence). Separating the data at the median luminosity of the \overline{z}=7.2 sample (M UV = -19.4) shows higher clustering at \overline{z}=5.9 for bright galaxies (r_0=5.5^{+1.4}_{-1.6}h^{-1}cMpc, b=6.2^{+1.2}_{-1.5}) compared to faint galaxies (r_0=1.9^{+1.1}_{-1.0}h^{-1}cMpc, b=2.7^{+1.2}_{-1.2}) implying a constant mass-to-light ratio { ({dlogM}/{dlogL)}} \sim 1.2^{+1.8}_{-0.8}. A similar trend is present in the \overline{z}=7.2 sample with larger uncertainty. Finally, our bias measurements allow us to investigate the fraction of dark matter halos hosting UV-bright galaxies (the duty cycle, epsilonDC). At \overline{z}=7.2 values near unity are preferred, which may be explained by the shortened halo assembly time at high redshift.

### No excess of bright galaxies around the redshift 7.1 quasar ULAS J1120+0641

Simpson, C., Mortlock, D., Warren, S., Cantalupo, S., Hewett, P., McLure, R., McMahon, R., and Venemans, B.

We present optical and near-infrared imaging of the field of the z = 7.0842 quasar ULAS J112001.48+064124.3 taken with the Hubble Space Telescope. We use these data to search for galaxies that may be physically associated with the quasar, using the Lyman break technique, and find three such objects, although the detection of one in Spitzer Space Telescope imaging strongly suggests it lies at z ˜ 2. This is consistent with the field luminosity function and indicates that there is no excess of >L★ galaxies within 1 Mpc of the quasar. A detection of the quasar shortwards of the Lyα line is consistent with the previously observed evolution of the intergalactic medium at z > 5.5.

### The colour distribution of galaxies at redshift five

Rogers, A. B., McLure, R. J., Dunlop, J. S., Bowler, R. A. A., Curtis-Lake, E. F., Dayal, P., Faber, S. M., Ferguson, H. C., Finkelstein, S. L., Grogin, N. A., Hathi, N. P., Kocevski, D., Koekemoer, A. M., and Kurczynski, P.

We present the results of a study investigating the rest-frame ultraviolet (UV) spectral slopes of redshift z ≈ 5 Lyman-break galaxies (LBGs). By combining deep Hubble Space Telescope imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and Hubble Ultra-Deep Field with ground-based imaging from the UKIRT Infrared Deep Sky Survey Ultra Deep Survey, we have produced a large sample of z ≈ 5 LBGs spanning an unprecedented factor of >100 in UV luminosity. Based on this sample we find a clear colour-magnitude relation (CMR) at z ≈ 5, such that the rest-frame UV slopes (β) of brighter galaxies are notably redder than their fainter counterparts. We determine that the z ≈ 5 CMR is well described by a linear relationship of the form: dβ = (-0.12 ± 0.02)dMUV, with no clear evidence for a change in CMR slope at faint magnitudes (i.e. MUV ≥ -18.9). Using the results of detailed simulations we are able, for the first time, to infer the intrinsic (i.e. free from noise) variation of galaxy colours around the CMR at z ≈ 5. We find significant (12σ) evidence for intrinsic colour variation in the sample as a whole. Our results also demonstrate that the width of the intrinsic UV slope distribution of z ≈ 5 galaxies increases from Δβ ≃ 0.1 at MUV = -18 to Δβ ≃ 0.4 at MUV = -21. We suggest that the increasing width of the intrinsic galaxy colour distribution and the CMR itself are both plausibly explained by a luminosity-independent lower limit of β ≈ -2.1, combined with an increase in the fraction of red galaxies in brighter UV-luminosity bins.

### The most metal-poor damped Lyman alpha systems: An insight into dwarf galaxies at high redshift

Cooke, R., Pettini, M., and Jorgenson, R. A.

In this paper, we analyze the kinematics, chemistry, and physical properties of a sample of the most metal-poor damped Lyman alpha systems (DLAs), to uncover their links to modern-day galaxies. We present evidence that the DLA population as a whole exhibits a knee' in the relative abundances of the alpha-capture and Fe-peak elements when the metallicity is [Fe/H] ~ -2.0. In this respect, the chemical evolution of DLAs is clearly different from that experienced by Milky Way halo stars, but resembles that of dwarf spheroidal galaxies in the Local Group. We also find a close correspondence between the kinematics of Local Group dwarf galaxies and of high redshift metal-poor DLAs, which further strengthens this connection. On the basis of such similarities, we propose that the most metal-poor DLAs provide us with a unique opportunity to directly study the dwarf galaxy population more than ten billion years in the past, at a time when many dwarf galaxies were forming the bulk of their stars. To this end, we have measured some of the key physical properties of the DLA gas, including their neutral gas mass, size, kinetic temperature, density, and turbulence. We find that metal-poor DLAs mostly consist of a warm neutral medium with T_gas ~ 9600 K predominantly held up by thermal pressure. Furthermore, all of the DLAs in our sample exhibit a subsonic turbulent Mach number, implying that the gas distribution is largely smooth. These results are among the first empirical descriptions of the environments where the first few generations of stars may have formed in our Universe.

### The Sizes of Candidate $z\sim9-10$ Galaxies; confirmation of the bright CANDELS sample and correlation with luminosity and mass

Holwerda, B. W., Bouwens, R., Oesch, P., Smit, R., Illingworth, G., and Labbe, I.

Recently, a small sample of six $z\sim9-10$ candidates was discovered in CANDELS that are $\sim10-20\times$ more luminous than any of the previous $z\sim9-10$ galaxies identified over the HUDF/XDF and CLASH fields. We test the nature of these sources by comparing their sizes with the expected sizes of luminous galaxies at $z\sim9-10$. Using galfit to derive sizes from the CANDELS F160W images of these candidates, we find a mean size of 0.16" $\pm$ 0.06" (or $0.6\pm0.3$ kpc at $z\sim9-10$). This is much smaller than the 0.59" mean size found for lower redshift IRAC-red interlopers, and handsomely matches the 0.16 (0.6 kpc) size expected from extrapolating lower redshift measurements to $z\sim9-10$. Assuming the bright sample is at $z\sim9-10$, we use this sample to extend current constraints on the size-luminosity, size-mass relation, and size evolution of galaxies to $z\sim10$. We find that the $z\sim9-10$ candidate galaxies have similar sizes and luminosities as their $z\sim7$ counterparts. They have star-formation-rate surface densities in the range of $\Sigma_{SFR} = 1-10 ~ M_\odot ~ yr^{-1} ~ kpc^{-2}$, similar to those measured for lower-redshift ($z = 6-8$). The stellar mass-size relation is uncertain, but also similar to those inferred for galaxies at z=6, 7 and 8. In combination with previous size measurements at z=4-7, we find a size evolution of $(1+z)^{-m}$ with $m = 1.0 \pm 0.1$ for $> 0.3L_* (z=3)$ galaxies, consistent with the evolution previously derived from $2 < z < 8$ galaxies.

### Properties of galaxies reproduced by a hydrodynamic simulation

Vogelsberger, M., Genel, S., Springel, V., Torrey, P., Sijacki, D., Xu, D., Snyder, G., Bird, S., Nelson, D., and Hernquist, L.

Previous simulations of the growth of cosmic structures have broadly reproduced the cosmic web' of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies, because of numerical inaccuracies and incomplete physical models. Moreover, they were unable to track the small-scale evolution of gas and stars to the present epoch within a representative portion of the Universe. Here we report a simulation that starts 12 million years after the Big Bang, and traces 13 billion years of cosmic evolution with 12 billion resolution elements in a cube of 106.5 megaparsecs a side. It yields a reasonable population of ellipticals and spirals, reproduces the observed distribution of galaxies in clusters and characteristics of hydrogen on large scales, and at the same time matches the `metal' and hydrogen content of galaxies on small scales.

### Cosmology: A virtual Universe

Boylan-Kolchin, M.

A numerical simulation of cosmic structure formation reproduces both large- and smaller-scale features of a representative volume of the Universe from early in its history to the present day. See Article p.177

### 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., Labbé, I., Smit, R., Franx, M., van Dokkum, P. G., Momcheva, I., Ashby, M. L. N., Fazio, G. G., Huang, J.-S., Willner, S. P., Gonzalez, V., Magee, D., Trenti, M., Brammer, G. B., Skelton, R. E., and Spitler, L. R.

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. Two similarly bright sources are also detected in a reanalysis of the GOODS-S data set. Three of the four galaxies in GOODS-N are significantly detected at 4.5σ-6.2σ in the very deep Spitzer/IRAC 4.5 μm data, as is one of the GOODS-S candidates. Furthermore, the brightest of our candidates (at z = 10.2 ± 0.4) is robustly detected also at 3.6 μm (6.9σ), revealing a flat UV spectral energy distribution with a slope β = -2.0 ± 0.2, consistent with demonstrated trends with luminosity at high redshift. Thorough testing and use of grism data excludes known low-redshift contamination at high significance, including single emission-line sources, but as-yet unknown low redshift sources could provide an alternative solution given the surprising luminosity of these candidates. Finding such bright galaxies at z ~ 9-10 suggests that the luminosity function for luminous galaxies might evolve in a complex way at z > 8. The cosmic star formation rate density still shows, however, an order-of-magnitude increase from z ~ 10 to z ~ 8 since the dominant contribution comes from low-luminosity sources. Based on the IRAC detections, we derive galaxy stellar masses at z ~ 10, finding that these luminous objects are typically 109 M &sun;. This allows for a first estimate of the cosmic stellar mass density at z ~ 10 resulting in log _{10}\rho _{*} = 4.7^{+0.5}_{-0.8} M &sun; Mpc-3 for galaxies brighter than M UV ~ -18. The remarkable brightness, and hence luminosity, of these z ~ 9-10 candidates will enable deep spectroscopy to determine their redshift and nature, and highlights the opportunity for the James Webb Space Telescope to map the buildup of galaxies at redshifts much earlier than z ~ 10. 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.

### The Luminosity Function at z ~ 8 from 97 Y-band Dropouts: Inferences about Reionization

Schmidt, K. B., Treu, T., Trenti, M., Bradley, L. D., Kelly, B. C., Oesch, P. A., Holwerda, B. W., Shull, J. M., and Stiavelli, M.

We present the largest search to date for Y-band dropout galaxies (z ~ 8 Lyman break galaxies, LBGs) based on 350 arcmin2 of Hubble Space Telescope observations in the V, Y, J, and H bands from the Brightest of Reionizing Galaxies (BoRG) survey. In addition to previously published data, the BoRG13 data set presented here includes approximately 50 arcmin2 of new data and deeper observations of two previous BoRG pointings, from which we present 9 new z ~ 8 LBG candidates, bringing the total number of BoRG Y-band dropouts to 38 with 25.5 <= mJ <= 27.6 (AB system). We introduce a new Bayesian formalism for estimating the galaxy luminosity function, which does not require binning (and thus smearing) of the data and includes a likelihood based on the formally correct binomial distribution as opposed to the often-used approximate Poisson distribution. We demonstrate the utility of the new method on a sample of 97 Y-band dropouts that combines the bright BoRG galaxies with the fainter sources published in Bouwens et al. from the Hubble Ultra Deep Field and Early Release Science programs. We show that the z ~ 8 luminosity function is well described by a Schechter function over its full dynamic range with a characteristic magnitude M^\star = -20.15^{+0.29}_{-0.38}, a faint-end slope of \alpha = -1.87^{+0.26}_{-0.26}, and a number density of log _{10} \phi ^\star [{Mpc}^{-3}] = -3.24^{+0.25}_{-0.24}. Integrated down to M = -17.7, this luminosity function yields a luminosity density log _{10} \epsilon [erg\, s^{-1\, Hz^{-1}\, Mpc^{-3}}] = 25.52^{+0.05}_{-0.05}. Our luminosity function analysis is consistent with previously published determinations within 1σ. The error analysis suggests that uncertainties on the faint-end slope are still too large to draw a firm conclusion about its evolution with redshift. We use our statistical framework to discuss the implication of our study for the physics of reionization. By assuming theoretically motivated priors on the clumping factor and the photon escape fraction we show that the UV luminosity density from galaxy samples down to M = -17.7 can ionize only 10%-50% of the neutral hydrogen at z ~ 8. Full reionization would require extending the luminosity function down to M = -15. The data are consistent with a substantial fraction of neutral hydrogen at z > 7, in agreement with recent suggestions based on deep spectroscopy of z ~ 8 LBGs.

### Frontier Fields: High-Redshift Predictions and Early Results

Coe, D., Bradley, L., and Zitrin, A.

The Frontier Fields program is obtaining deep Hubble and Spitzer Space Telescope images of new "blank" fields and nearby fields gravitationally lensed by massive galaxy clusters. The Hubble images of the lensed fields are revealing nJy sources (AB mag > 31), the faintest galaxies yet observed. In this paper, we present high-redshift (z > 6) number count predictions for the full program and candidates in three of the first Hubble Frontier Fields images. The full program will transform our understanding of galaxy evolution in the first 600 million years (z > 9). Where previous programs yielded perhaps a dozen z > 9 candidates, the Frontier Fields may yield ~70 (~6 per field). We base this estimate on an extrapolation of luminosity functions observed between 4 < z < 8 and gravitational lensing models submitted by the community. However, in the first two deep infrared Hubble images obtained to date, we find z ~ 8 candidates but no strong candidates at z > 9. This might suggest a deficit of faint z > 9 galaxies as also reported in the Ultra Deep Field (even while excesses of brighter z > 9 galaxies were reported in shallower fields). At these redshifts, cosmic variance (field-to-field variation) is expected to be significant (greater than +/-50%) and include clustering of early galaxies formed in overdensities. The full Frontier Fields program will significantly mitigate this uncertainty by observing six independent sightlines each with a lensing cluster and nearby blank field.

### When VLT Meets HST: The HUGS Survey

Fontana, A., Dunlop, J. S., Paris, D., Targett, T., Boutsia, K., Castellano, M., Galametz, A., Grazian, A., McLure, R., Merlin, E., Pentericci, L., Wuyts, S., Almaini, O., Caputi, K., Chary, R.-R., Cirasuolo, M., Conselice, C., Cooray, A., Daddi, E., Dickinson, M., Faber, S. M., Fazio, G., Ferguson, H., Giallongo, E., Giavalisco, M., Grogin, N., Hathi, N., Koekemoer, A., Koo, D. C., Lucas, R., Nonino, M., Rix, H.-W., Renzini, A., Rosario, D., Santini, P., Scarlata, C., Sommariva, V., Stark, D. P., van der Wel, A., Vanzella, E., Wild, V., Yan, H., and Zibetti, S.

A new ultra-deep near-infrared imaging survey has been completed using the HAWK-I imager at the VLT. It is named HUGS (HAWK-I Ultra Deep Survey and GOODS Survey) and delivers the deepest, highest quality images ever collected in the K-band. HUGS complements the data delivered by the HST CANDELS survey over two well-studied extragalactic fields, and promises to open up exciting new opportunities to explore the highest redshift Universe. The survey is outlined and faint galaxy number counts and the search for passive galaxies in the early Universe are highlighted. The HUGS data have been completely analysed and are being made public to the worldwide community.

### High-redshift galaxies and low-mass stars

Wilkins, S. M., Stanway, E. R., and Bremer, M. N.

The sensitivity available to near-infrared surveys has recently allowed us to probe the galaxy population at z ≈ 7 and beyond. The existing Hubble Wide Field Camera 3 (WFC3) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond 1 μm - a capability that will be further extended with the launch and commissioning of the James Webb Space Telescope (JWST). As new regions of parameter space in both colour and depth are probed, new challenges for distant galaxy surveys are identified. In this paper, we present an analysis of the colours of L- and T-dwarf stars in widely used photometric systems. We also consider the implications of the newly identified Y-dwarf population - stars that are still cooler and less massive than T-dwarfs for both the photometric selection and spectroscopic follow-up of faint and distant galaxies. We highlight the dangers of working in the low-signal-to-noise regime, and the potential contamination of existing and future samples. We find that Hubble/WFC3 and VISTA/VIRCam Y-drop selections targeting galaxies at z ˜ 7.5 are vulnerable to contamination from T- and Y-class stars. Future observations using JWST, targeting the z ˜ 7 galaxy population, are also likely to prove difficult without deep medium-band observations. We demonstrate that single emission line detections in typical low-signal-to-noise spectroscopic observations may also be suspect, due to the unusual spectral characteristics of the cool dwarf star population.

### Star-forming blue ETGs in two newly discovered galaxy overdensities in the HUDF at z=1.84 and 1.9: unveiling the progenitors of passive ETGs in cluster cores

Mei, S., Scarlata, C., Pentericci, L., Newman, J. A., Weiner, B. J., Ashby, M. L. N., Castellano, M., Conselice, C. J., Finkelstein, S. L., Galametz, A., Grogin, N. A., Koekemoer, A. M., Huertas-Company, M., Lani, C., Lucas, R. A., Papovich, C., Rafelski, M., and Tepliz, H. I.

We present the discovery of two galaxy overdensities in the HST UDF: a proto-cluster, HUDFJ0332.4-2746.6 at $z = 1.84 \pm 0.01$, and a group, HUDFJ0332.5-2747.3 at $z =1.90 \pm 0.01$. The velocity dispersion of HUDFJ0332.4-2746.6 implies a mass of $M_{200}= (2.2 \pm 1.8) \times 10^{14} M_{\odot}$, consistent with the lack of extended X-ray emission. Neither overdensity shows evidence of a red sequence. About $50\%$ of their members show interactions and/or disturbed morphologies, which are a signature of merger remnants. Most of their morphologically classified ETGs have blue colors and show recent star-formation. These observations reveal for the first time large fractions of spectroscopically confirmed star-forming blue ETGs in proto-clusters at $z\approx 2$. These star-forming ETGs are most likely among the progenitors of the quiescent population in clusters at more recent epochs. Their mass-size relation is consistent with that of passive ETGs in clusters at $z\sim0.7-1.5$. If these galaxies are the progenitors of cluster ETGs at these lower redshifts, their size would evolve according to a similar mass-size relation. It is noteworthy that quiescent ETGs in clusters at $z=1.8-2$ also do not show any significant size evolution over this redshift range, contrary to field ETGs. The ETG fraction of our sample is $\lesssim 40\%$, compared to the typical quiescent ETG fraction of $\approx 80\%$ in cluster cores at $z< 1$. The fraction, masses and colors of the newly discovered ETGs imply that other cluster ETGs will be formed/accreted at later time.

### UV Luminosity Functions at redshifts z~4 to z~10: 10000 Galaxies from HST Legacy Fields

Bouwens, R. J., Illingworth, G. D., Oesch, P. A., Trenti, M., Labbe', I., Bradley, L., Carollo, M., van Dokkum, P. G., Gonzalez, V., Holwerda, B., Franx, M., Spitler, L., Smit, R., and Magee, D.

The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map out the evolution of the UV LF from z~10 to z~4. We develop new color criteria that more optimally utilize the full wavelength coverage from the optical+near-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3002, 857, 481, 217, and 6 galaxy candidates at z~4, z~5, z~6, z~7, z~8, and z~10, respectively from the ~1000 arcmin**2 area covered by these datasets. The selection of z~4-8 galaxies over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are apparent at z>=7. Our new LF determinations at z~4 and z~5 span a 6-mag baseline (-22.5 to -16 AB mag). These determinations agree well with previous estimates, but the larger samples and the larger volumes probed here result in a more reliable sampling of >L* galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to 3.4 sigma significance for a steeper faint-end slope to the UV LF at z>4, with alpha evolving from alpha=-1.64+/-0.04 at z~4 to alpha=-2.06+/-0.13 at z~7. The observed steepening of the UV LF is consistent with that expected from the evolution of the halo mass function. With our improved constraints at the bright end, we find less evolution in the characteristic luminosity M* over the redshift range z~4 to z~7 consistent with current models; the observed evolution in the LF is now largely represented by changes in phi*. Even with the much larger sample of bright galaxies, we find no evidence for the LF having a non-Schechter-like form at z~4-8. A simple conditional LF model based on halo growth and a modest evolution in the M/L ratio of halos provides a good representation of the evolution of the UV LF.

### The 2012 Hubble Ultra Deep Field (UDF12): Observational Overview

Koekemoer, A. M., Ellis, R. S., McLure, R. J., Dunlop, J. S., Robertson, B. E., Ono, Y., Schenker, M. A., Ouchi, M., Bowler, R. A. A., Rogers, A. B., Curtis-Lake, E., Schneider, E., Charlot, S., Stark, D. P., Furlanetto, S. R., Cirasuolo, M., Wild, V., and Targett, T.

We present the 2012 Hubble Ultra Deep Field campaign (UDF12), a large 128 orbit Cycle 19 Hubble Space Telescope program aimed at extending previous Wide Field Camera 3 (WFC3)/IR observations of the UDF by quadrupling the exposure time in the F105W filter, imaging in an additional F140W filter, and extending the F160W exposure time by 50%, as well as adding an extremely deep parallel field with the Advanced Camera for Surveys (ACS) in the F814W filter with a total exposure time of 128 orbits. The principal scientific goal of this project is to determine whether galaxies reionized the universe; our observations are designed to provide a robust determination of the star formation density at z >~ 8, improve measurements of the ultraviolet continuum slope at z ~ 7-8, facilitate the construction of new samples of z ~ 9-10 candidates, and enable the detection of sources up to z ~ 12. For this project we committed to combining these and other WFC3/IR imaging observations of the UDF area into a single homogeneous dataset to provide the deepest near-infrared observations of the sky. In this paper we present the observational overview of the project and describe the procedures used in reducing the data as well as the final products that were produced. We present the details of several special procedures that we implemented to correct calibration issues in the data for both the WFC3/IR observations of the main UDF field and our deep 128 orbit ACS/WFC F814W parallel field image, including treatment for persistence, correction for time-variable sky backgrounds, and astrometric alignment to an accuracy of a few milliarcseconds. We release the full, combined mosaics comprising a single, unified set of mosaics of the UDF, providing the deepest near-infrared blank-field view of the universe currently achievable, reaching magnitudes as deep as AB ~ 30 mag in the near-infrared, and yielding a legacy dataset on this field.

### Clustering at high redshift: the connection between Lyman α emitters and Lyman break galaxies

Jose, C., Srianand, R., and Subramanian, K.

We present a physically motivated semi-analytic model to understand the clustering of high-redshift Lyman α emitters (LAEs). We show that the model parameters constrained by the observed luminosity functions can be used to predict large-scale bias and angular correlation function of LAEs. These predictions are shown to reproduce the observations remarkably well. We find that average masses of dark matter haloes hosting LAEs brighter than the threshold narrow-band magnitude ˜25 are ˜1011 M&sun;. These are smaller than that of typical Lyman break galaxies (LBGs) brighter than a similar threshold continuum magnitude by a factor of ˜10. This results in a smaller clustering strength of LAEs compared to LBGs. However, using the observed relationship between the UV continuum and Lyman α luminosity of LAEs, we show that both LAEs and LBGs belong to the same parent galaxy population with narrow-band techniques having greater efficiency in picking up galaxies with low UV luminosity. We also show that the lack of evidence for the presence of the one-halo term in the observed LAE angular correlation functions can be attributed to a sub-Poisson distribution of LAEs in dark matter haloes as a result of their low halo occupations.

### Low Masses and High Redshifts: The Evolution of the Mass-Metallicity Relation

Henry, A., Scarlata, C., Domínguez, A., Malkan, M., Martin, C. L., Siana, B., Atek, H., Bedregal, A. G., Colbert, J. W., Rafelski, M., Ross, N., Teplitz, H., Bunker, A. J., Dressler, A., Hathi, N., Masters, D., McCarthy, P., and Straughn, A.

We present the first robust measurement of the high redshift mass-metallicity (MZ) relation at 108 <~ M/M &sun; <~ 1010, obtained by stacking spectra of 83 emission-line galaxies with secure redshifts between 1.3 <~ z <~ 2.3. For these redshifts, infrared grism spectroscopy with the Hubble Space Telescope Wide Field Camera 3 is sensitive to the R 23 metallicity diagnostic: ([O II] λλ3726, 3729 + [O III] λλ4959, 5007)/Hβ. Using spectra stacked in four mass quartiles, we find a MZ relation that declines significantly with decreasing mass, extending from 12+log(O/H) = 8.8 at M = 109.8 M &sun;, to 12+log(O/H) = 8.2 at M = 108.2 M &sun;. After correcting for systematic offsets between metallicity indicators, we compare our MZ relation to measurements from the stacked spectra of galaxies with M >~ 109.5 M &sun; and z ~ 2.3. Within the statistical uncertainties, our MZ relation agrees with the z ~ 2.3 result, particularly since our somewhat higher metallicities (by around 0.1 dex) are qualitatively consistent with the lower mean redshift (z = 1.76) of our sample. For the masses probed by our data, the MZ relation shows a steep slope which is suggestive of feedback from energy-driven winds, and a cosmological downsizing evolution where high mass galaxies reach the local MZ relation at earlier times. In addition, we show that our sample falls on an extrapolation of the star-forming main sequence (the SFR-M * relation) at this redshift. This result indicates that grism emission-line selected samples do not have preferentially high star formation rates (SFRs). Finally, we report no evidence for evolution of the mass-metallicity-SFR plane; our stack-averaged measurements show excellent agreement with the local relation. 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.

### 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.