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The advent of the JWST has revolutionised our understanding of high-redshift galaxies. In particular, the NIRCam instrument on-board JWST has revealed a population of Hubble Space Telescope (HST)-dark galaxies that had previously evaded optical detection, potentially due to significant dust obscuration, quiescence, or simply extreme redshift. Here, we present the first NIRSpec spectra of 23 HST-dark galaxies ($\mathrm{H-F444W>1.75}$), unveiling their nature and physical properties. This sample includes both dusty and quiescent galaxies with spectroscopic data from NIRSpec/PRISM, providing accurate spectroscopic redshifts with $\mathrm{\overline{z}_{spec} = 4.1 \pm 0.7}$. The spectral features demonstrate that, while the majority of HST-dark galaxies are dusty, a substantial fraction, $\mathrm{13^{+9}_{-6} \%}$, are quiescent. For the dusty galaxies, we have quantified the dust attenuation using the Balmer decrement ($\mathrm{H\alpha / H\beta}$), finding attenuations $\mathrm{A_{V} > 2\ mag}$. We find that HST-dark dusty galaxies are $\mathrm{H\alpha}$ emitters with equivalent widths spanning the range $\mathrm{ 68 A < EW_{H\alpha} < 550 A }$, indicative of a wide range of recent star-formation activity. Whether dusty or quiescent, we find that HST-dark galaxies are predominantly massive, with 85\% of the galaxies in the sample having masses $\mathrm{log(M_{*}/M_{\odot}) > 9.8}$. This pilot NIRSpec program reveals the diverse nature of HST-dark galaxies and highlights the effectiveness of NIRSpec/PRISM spectroscopic follow-up in distinguishing between dusty and quiescent galaxies and properly quantifying their physical properties. Upcoming research utilising higher-resolution NIRSpec data and combining JWST with ALMA observations will enhance our understanding of these enigmatic and challenging sources., Comment: 13 pages, 11 figures

We present a new determination of the evolving galaxy UV luminosity function (LF) over the redshift range $8.58.5)>0.05$) to undertake a statistical calculation of the evolving UV LF. Our new measurements span $\simeq4$ magnitudes in UV luminosity at $z=9-12.5$, placing new constraints on both the shape and evolution of the LF at early times. We fit our observational data-points with a double-power law (DPL) function and explore the evolution of the DPL parameters. Our UV LF measurements yield a new estimate of the early evolution of cosmic star-formation rate density ($\rho_{\rm{SFR}}$) which confirms the gradual, log-linear decline deduced from early JWST studies, at least out to $z \simeq 12$. Finally we show that the observed early evolution of the galaxy UV LF (and $\rho_{\rm{SFR}}$) can be reproduced in a ${\rm \Lambda}$CDM Universe, with no change in dust properties or star-formation efficiency required out to $z \simeq 12$. Instead, we show that a progressive trend towards younger stellar population ages can reproduce the observations, and we show that the typical ages required at $z \simeq$ 8, 9, 10, and 11 all converge on a time $\simeq 380-330$ Myr after the Big Bang, indicative of a rapid emergence of early galaxies at $z \simeq 12 - 13$. This is consistent with the first indications of a steeper drop-off in the galaxy population we find beyond $z \simeq 13$, possibly reflecting the rapid evolution of the halo mass function at earlier times., Comment: 15 pages, 9 figures, submitted to MNRAS

The emergence of massive, optically-faint galaxies in infrared observations has revealed that our view of the high-redshift Universe was previously incomplete. With the advent of JWST, we can for the first time probe the rest-frame optical emission of galaxies at $z>3$ with high sensitivity and spatial resolution, thus moving towards a more complete census of the galaxy population at high redshifts. To this end, we present a sample of 148 massive, dusty galaxies from the JWST/CEERS survey, colour-selected using solely JWST bands. With deep JWST/NIRCam data from 1.15$\mu$m to 4.44$\mu$m and ancillary HST/ACS and WFC3 data, we determine the physical properties of our sample using spectral energy distribution fitting with BAGPIPES. We demonstrate that our selection method efficiently identifies massive ($\mathrm{\langle \log M_\star/M_\odot \rangle \sim 10}$) and dusty ($\mathrm{\langle A_V\rangle \sim 2.7\ mag}$) sources, with a majority at $z>3$ and predominantly lying on the galaxy main-sequence. The main results of this work are the stellar mass functions (SMF) of red, optically-faint galaxies from redshifts between $39.25}$ may have been underestimated by $\sim$20-25% at $z\sim3-6$, and $\sim$110% at $z\sim6-8$., Comment: 19 pages, 10 figures, submitted to MNRAS

We present results of dust continuum and [CII]$\,158\,{\rm \mu m}$ emission line observations of a remarkably UV-luminous ($M_{\rm UV}=-21.6$) galaxy at $z=10.603$: GN-z11. Using the Northern Extended Millimeter Array (NOEMA), observations have been carried out over multiple observing cycles. We achieved a high sensitivity resulting in a $\lambda_{\rm rest}=160\,{\rm \mu m}$ continuum $1\,\sigma$ depth of $13.0\,\rm{\mu Jy/beam}$ and a [CII] emission line $1\,\sigma$ sensitivity of $31\,\rm{mJy/beam\,km/s}$ using $50\,\rm{km/s}$ binning with a $\sim 2\,{\rm arcsec}$ synthesized beam. Neither dust continuum nor [CII]$\,158\,{\rm \mu m}$ line emission are detected at the expected frequency of $\nu_{\rm [CII]} = 163.791\,\rm{GHz}$ and the sky location of GN-z11. The upper limits show that GN-z11 is neither luminous in $L_{\rm IR}$ nor $L_{\rm [CII]}$, with a dust mass $3\,\sigma$ limit of ${\rm log}(M_{\rm dust}/{\rm M_{\odot}}) < 6.5-6.9$ and with a [CII] based molecular gas mass $3\,\sigma$ limit of ${\rm log}(M_{\rm mol,[CII]}/{\rm M_{\odot}}) < 9.3$. Together with radiative transfer calculations, we also investigated the possible cause of the dust poor nature of the GN-z11 showed by the blue color in the UV continuum of GN-z11 ($\beta_{\rm UV}=-2.4$), and found that $\gtrsim3\times$ deeper observations are crucial to study dust production at very high-redshift. Nevertheless, our observations show the crucial role of deep mm/submm observations of very high redshift galaxies to constrain multiple phases in the interstellar medium., Comment: submitted to MNRAS, 7 pages, 4 figures, 1 table

We present the JWST Cycle 1 53.8hr medium program FRESCO, short for "First Reionization Epoch Spectroscopically Complete Observations". FRESCO covers 62 arcmin$^2$ in each of the two GOODS/CANDELS fields for a total area of 124 arcmin$^2$ exploiting JWST's powerful new grism spectroscopic capabilities at near-infrared wavelengths. By obtaining ~2 hr deep NIRCam/grism observations with the F444W filter, FRESCO yields unprecedented spectra at R~1600 covering 3.8 to 5.0 $\mu$m for most galaxies in the NIRCam field-of-view. This setup enables emission line measurements over most of cosmic history, from strong PAH lines at z~0.2-0.5, to Pa$\alpha$ and Pa$\beta$ at z~1-3, HeI and [SIII] at z~2.5-4.5, H$\alpha$ and [NII] at z~5-6.5, up to [OIII] and H$\beta$ for z~7-9 galaxies, and possibly even [OII] at z~10-12. FRESCO's grism observations provide total line fluxes for accurately estimating galaxy stellar masses and calibrating slit-loss corrections of NIRSpec/MSA spectra in the same field. Additionally, FRESCO results in a mosaic of F182M, F210M, and F444W imaging in the same fields to a depth of ~28.2 mag (5 $\sigma$ in 0.32" diameter apertures). Together with this publication, the v1 imaging mosaics are released as high-level science products via MAST. Here, we describe the overall survey design and the key science goals that can be addressed with FRESCO. We also highlight several, early science results, including: spectroscopic redshifts of Lyman break galaxies that were identified almost 20 years ago, the discovery of broad-line active galactic nuclei at z>4, and resolved Pa$\alpha$ maps of galaxies at z~1.4. These results demonstrate the enormous power for serendipitous discovery of NIRCam/grism observations., Comment: 12 pages, 11 figures; MNRAS in press; for more information on the survey and data releases, see http://jwst-fresco.astro.unige.ch/ and https://archive.stsci.edu/hlsp/fresco

In recent years, a number of Lyman continuum (LyC) leaker candidates at intermediate redshifts have been found, providing insight into how the Universe was reionised at early cosmic times. Here we identify new LyC leaker candidates at $z\approx 3-4.5$ and compare them to objects from the literature to get an overview of the different observed escape fractions and their relation to the properties of the Lyman $\alpha$ (Ly$\alpha$) emission line. The aim of this work is to test indicators for LyC leakage and to improve our understanding of the kind of galaxies from which LyC radiation can escape. We use data from the Hubble Deep Ultraviolet (HDUV) legacy survey to search for LyC emission based on a sample of $\approx 2000$ Ly$\alpha$ emitters (LAEs) detected previously in two surveys with the Multi-Unit Spectroscopic Explorer (MUSE), MUSE-Deep and MUSE-Wide. Based on their known redshifts and positions, we look for potential LyC leakage in the WFC3/UVIS F336W band of the HDUV. The escape fractions are measured and compared based on spectral energy distribution (SED) fitting performed using the CIGALE software. We add twelve objects to the sample of known LyC leaker candidates, one of which was previously known, and compare their Ly$\alpha$ properties to their escape fractions. We find escape fractions between $\sim 20\%$ and $\sim 90\%$, assuming a high transmission in the intergalactic medium (IGM). We show a method to use the number of LyC leaker candidates we find to infer the underlying average escape fraction of galaxies, which is $\approx 12\%$. Based on their Ly$\alpha$ properties, we conclude that LyC leakers are not very different from other high-z LAEs and suggest that most LAEs could be leaking LyC even if this can not always be detected due to the direction of emission and the transmission properties of the IGM.

We assemble a large comprehensive sample of 2534 z~2, 3, 4, 5, 6, 7, 8, and 9 galaxies lensed by the six clusters from the Hubble Frontier Fields (HFF) program. Making use of the availability of multiple independent magnification models for each of the HFF clusters and alternatively treating one of the models as the "truth," we show that the median magnification factors from the v4 parametric models are typically reliable to values of 30 to 50, and in one case to 100. Using the median magnification factor from the latest v4 models, we estimate the UV luminosities of the 2534 lensed z~2-9 galaxies, finding sources as faint as -12.4 mag at z~3 and -12.9 mag at z~7. We explicitly demonstrate the power of the surface density-magnification relations Sigma(z) vs. mu in the HFF clusters to constrain both distant galaxy properties and cluster lensing properties. Based on the Sigma(z) vs. mu relations, we show that the median magnification estimates from existing public models must be reliable predictors of the true magnification mu to mu<15 (95% confidence). We also use the observed Sigma(z) vs. mu relations to derive constraints on the evolution of the luminosity function faint-end slope from z~7 to z~2, showing that faint-end slope results can be consistent with blank-field studies if, and only if, the selection efficiency shows no strong dependence on the magnification factor mu. This can only be the case if very low luminosity galaxies are very small, being unresolved in deep lensing probes., Comment: 22 pages, 13 figures, 6 tables, accepted for publication in the Astrophysical Journal

We present new determinations of the rest-UV luminosity functions (LFs) at z = 2-9 to extremely low luminosities (>-14 mag) from a sample of >2500 lensed galaxies found behind the Hubble Frontier Fields (HFF) clusters. For the first time, we present faint-end slope results from lensed samples that are fully consistent with blank-field results over the redshift range z = 2-9, while reaching to much lower luminosities than possible from the blank-field studies. Combining the deep lensed sample with the large blank-field samples allows us to set tight constraints on the faint-end slope alpha of the z = 2-9 UV LFs and its evolution. We find a smooth flattening in alpha from -2.28 +/- 0.10 (z = 9) to -1.53 +/- 0.03 (z = 2) with cosmic time (d alpha/dz = -0.11 +/- 0.01), fully consistent with dark matter halo buildup. We utilize these new results to present new measurements of the evolution in the UV luminosity density rho ( UV ) brighter than -13 mag from z similar to 9 to z similar to 2. Accounting for the star formation rate (SFR) densities to faint luminosities implied by our LF results, we find that unobscured star formation dominates the SFR density at z greater than or similar to 4, with obscured star formation dominant thereafter. Having shown we can quantify the faint-end slope alpha of the LF accurately with our lensed HFF samples, we also quantify the apparent curvature in the shape of the UV LF through a curvature parameter delta. The constraints on the curvature delta strongly rule out the presence of a turn-over brighter than -13.1 mag at z similar to 3, -14.3 mag at z similar to 6, and -15.5 mag at all other redshifts between z similar to 9 and z similar to 2.

We present new determinations of the rest-UV luminosity functions (LFs) at z = 2-9 to extremely low luminosities (>-14 mag) from a sample of >2500 lensed galaxies found behind the Hubble Frontier Fields (HFF) clusters. For the first time, we present faint-end slope results from lensed samples that are fully consistent with blank-field results over the redshift range z = 2-9, while reaching to much lower luminosities than possible from the blank-field studies. Combining the deep lensed sample with the large blank-field samples allows us to set tight constraints on the faint-end slope alpha of the z = 2-9 UV LFs and its evolution. We find a smooth flattening in alpha from -2.28 +/- 0.10 (z = 9) to -1.53 +/- 0.03 (z = 2) with cosmic time (d alpha/dz = -0.11 +/- 0.01), fully consistent with dark matter halo buildup. We utilize these new results to present new measurements of the evolution in the UV luminosity density rho ( UV ) brighter than -13 mag from z similar to 9 to z similar to 2. Accounting for the star formation rate (SFR) densities to faint luminosities implied by our LF results, we find that unobscured star formation dominates the SFR density at z greater than or similar to 4, with obscured star formation dominant thereafter. Having shown we can quantify the faint-end slope alpha of the LF accurately with our lensed HFF samples, we also quantify the apparent curvature in the shape of the UV LF through a curvature parameter delta. The constraints on the curvature delta strongly rule out the presence of a turn-over brighter than -13.1 mag at z similar to 3, -14.3 mag at z similar to 6, and -15.5 mag at all other redshifts between z similar to 9 and z similar to 2.

We assemble a large comprehensive sample of 2534 z similar to 2, 3, 4, 5, 6, 7, 8, and 9 galaxies lensed by the six clusters from the Hubble Frontier Fields (HFF) program. Making use of the availability of multiple independent magnification models for each of the HFF clusters and alternatively treating one of the models as the "truth," we show that the median magnification factors from the v4 parametric models are typically reliable to values of 30-50, and in one case to 100. Using the median magnification factor from the latest v4 models, we estimate the UV luminosities of the 2534 lensed z similar to 2-9 galaxies, finding sources as faint as -12.4 mag at z similar to 3 and -12.9 mag at z similar to 7. We explicitly demonstrate the power of the surface density-magnification relations sigma(z) versus mu in the HFF clusters to constrain both distant galaxy properties and cluster lensing properties. Based on the sigma(z) versus mu relations, we show that the median magnification estimates from existing public models must be reliable predictors of the true magnification mu to mu < 15 (95% confidence). We also use the observed sigma(z) versus mu relations to derive constraints on the evolution of the luminosity function faint-end slope from z similar to 7 to z similar to 2, showing that faint-end slope results can be consistent with blank-field studies if, and only if, the selection efficiency shows no strong dependence on the magnification factor mu. This can only be the case if very low-luminosity galaxies are very small, being unresolved in deep lensing probes.

Over the last few years, both ALMA and Spitzer/IRAC observations have revealed a population of likely massive galaxies at $z>3$ that was too faint to be detected in HST rest-frame ultraviolet imaging. However, due to the very limited photometry for individual galaxies, the true nature of these so-called HST-dark galaxies has remained elusive. Here, we present the first sample of such galaxies observed with very deep, high-resolution NIRCam imaging from the Early Release Science Program CEERS. 30 HST-dark sources are selected based on their red colours across 1.6 $\mu$m to 4.4 $\mu$m. Their physical properties are derived from 12-band multi-wavelength photometry, including ancillary HST imaging. We find that these galaxies are generally heavily dust-obscured ($A_{V}\sim2$ mag), massive ($\log (M/M_{\odot}) \sim10$), star-forming sources at $z\sim2-8$ with an observed surface density of $\sim0.8$ arcmin$^{-2}$. This suggests that an important fraction of massive galaxies may have been missing from our cosmic census at $z>3$ all the way into the Reionization epoch. The HST-dark sources lie on the main sequence of galaxies and add an obscured star formation rate density (SFRD) of $\mathrm{3.2^{+1.8}_{-1.3} \times 10^{-3} M_{\odot}/yr/Mpc^{3}}$ at $z\sim7$ showing likely presence of dust in the Epoch of Reionization. Our analysis shows the unique power of JWST to reveal this previously missing galaxy population and to provide a more complete census of galaxies at $z=2-8$ based on rest-frame optical imaging., Comment: 9 pages, 7 figures, accepted to MNRAS

We present new determinations of the rest-UV luminosity functions (LFs) at z=2-9 to extremely low luminosities (>-14 mag) from a sample of >2500 lensed galaxies found behind the HFF clusters. For the first time, we present faint-end slope results from lensed samples that are fully consistent with blank-field results over the redshift range z=2-9, while reaching to much lower luminosities than possible from the blank-field studies. Combining the deep lensed sample with the large blank-field samples allows us to set the tight constraints on the faint-end slope alpha of the z=2-9 UV LFs and its evolution. We find a smooth flattening in alpha from -2.28+/-0.10 (z=9) to -1.53+/-0.03 (z=2) with cosmic time (d(alpha)/dz=-0.11+/-0.01), fully consistent with dark matter halo buildup. We utilize these new results to present new measurements of the evolution in the UV luminosity density rho(UV) brightward of -13 mag from z~9 to z~2. Accounting for the SFR densities to faint luminosities implied by our LF results, we find that unobscured star formation dominates the SFR density at z>~4, with obscured star formation dominant thereafter. Having shown we can quantify the faint-end slope alpha of the LF accurately with our lensed HFF samples, we also quantify the apparent curvature in the shape of the UV LF through a curvature parameter delta. The constraints on the curvature delta strongly rule out the presence of a turn-over brightward of -13.1 mag at z~3, -14.3 mag at z~6, and -15.5 mag at all other redshifts between z~9 to z~2., Comment: 32 pages, 16 figures, 7 tables, ApJ, in press

We assemble a large comprehensive sample of 2534 z~2, 3, 4, 5, 6, 7, 8, and 9 galaxies lensed by the six clusters from the Hubble Frontier Fields (HFF) program. Making use of the availability of multiple independent magnification models for each of the HFF clusters and alternatively treating one of the models as the "truth," we show that the median magnification factors from the v4 parametric models are typically reliable to values of 30 to 50, and in one case to 100. Using the median magnification factor from the latest v4 models, we estimate the UV luminosities of the 2534 lensed z~2-9 galaxies, finding sources as faint as -12.4 mag at z~3 and -12.9 mag at z~7. We explicitly demonstrate the power of the surface density-magnification relations Sigma(z) vs. mu in the HFF clusters to constrain both distant galaxy properties and cluster lensing properties. Based on the Sigma(z) vs. mu relations, we show that the median magnification estimates from existing public models must be reliable predictors of the true magnification mu to mu<15 (95% confidence). We also use the observed Sigma(z) vs. mu relations to derive constraints on the evolution of the luminosity function faint-end slope from z~7 to z~2, showing that faint-end slope results can be consistent with blank-field studies if, and only if, the selection efficiency shows no strong dependence on the magnification factor mu. This can only be the case if very low luminosity galaxies are very small, being unresolved in deep lensing probes., Comment: 22 pages, 13 figures, 6 tables, accepted for publication in the Astrophysical Journal, updated to be consistent with the proofs

We assemble a large comprehensive sample of 2534 z similar to 2, 3, 4, 5, 6, 7, 8, and 9 galaxies lensed by the six clusters from the Hubble Frontier Fields (HFF) program. Making use of the availability of multiple independent magnification models for each of the HFF clusters and alternatively treating one of the models as the "truth," we show that the median magnification factors from the v4 parametric models are typically reliable to values of 30-50, and in one case to 100. Using the median magnification factor from the latest v4 models, we estimate the UV luminosities of the 2534 lensed z similar to 2-9 galaxies, finding sources as faint as -12.4 mag at z similar to 3 and -12.9 mag at z similar to 7. We explicitly demonstrate the power of the surface density-magnification relations sigma(z) versus mu in the HFF clusters to constrain both distant galaxy properties and cluster lensing properties. Based on the sigma(z) versus mu relations, we show that the median magnification estimates from existing public models must be reliable predictors of the true magnification mu to mu < 15 (95% confidence). We also use the observed sigma(z) versus mu relations to derive constraints on the evolution of the luminosity function faint-end slope from z similar to 7 to z similar to 2, showing that faint-end slope results can be consistent with blank-field studies if, and only if, the selection efficiency shows no strong dependence on the magnification factor mu. This can only be the case if very low-luminosity galaxies are very small, being unresolved in deep lensing probes.

We constrain the rest-UV size-luminosity relation for star-forming galaxies at z similar to 4 and z similar to 6, 7, and 8 identified behind clusters from the Hubble Frontier Fields (HFF) program. The size-luminosity relation is key to deriving accurate luminosity functions (LF) for faint galaxies. Making use of the latest lensing models and full data set for these clusters, lensing-corrected sizes and luminosities are derived for 68 z similar to 4, 184 z similar to 6, 93 z similar to 7, and 53 z similar to 8 galaxies. We show that size measurements can be reliably measured up to linear magnifications of similar to 30x, where the lensing models are well calibrated. The sizes we measure span a >1 dex range, from

We constrain the rest-UV size-luminosity relation for star-forming galaxies at z similar to 4 and z similar to 6, 7, and 8 identified behind clusters from the Hubble Frontier Fields (HFF) program. The size-luminosity relation is key to deriving accurate luminosity functions (LF) for faint galaxies. Making use of the latest lensing models and full data set for these clusters, lensing-corrected sizes and luminosities are derived for 68 z similar to 4, 184 z similar to 6, 93 z similar to 7, and 53 z similar to 8 galaxies. We show that size measurements can be reliably measured up to linear magnifications of similar to 30x, where the lensing models are well calibrated. The sizes we measure span a >1 dex range, from

Before the end of the epoch of reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly$\alpha$ lines of $z\gtrsim6$ galaxies in the intergalactic medium. Nevertheless, Ly$\alpha$ has been detected up to very high redshifts ($z\sim9$) for several especially UV luminous galaxies. Here, we test to what extent the galaxy's local environment might impact the Ly$\alpha$ transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly$\alpha$ emitters: EGS-zs8-1, EGS-zs8-2 and EGSY-z8p7 at $z_\mathrm{spec}=7.73$, 7.48, and 8.68, respectively. We combine the multi-wavelength HST imaging with Spitzer data to reliably select $z\sim7-9$ galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor $\sim 3-9\times$). Our analysis thus reveals ubiquitous overdensities around luminous Ly$\alpha$ emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. JWST follow-up observations of the neighbouring galaxies identified here will be needed to confirm their physical association and to map out the ionized regions produced by these sources., Comment: 13 pages, 14 figures, Submitted to MNRAS

We constrain the rest-UV size-luminosity relation for star-forming galaxies at z~4 and z~6, 7, and 8 identified behind clusters from the Hubble Frontier Fields (HFF) program. The size-luminosity relation is key to deriving accurate luminosity functions (LF) for faint galaxies. Making use of the latest lensing models and full data set for these clusters, lensing-corrected sizes and luminosities are derived for 68 z~4, 184 z~6, 93 z~7, and 53 z~8 galaxies. We show that size measurements can be reliably measured up to linear magnifications of 30x, where the lensing models are well calibrated. The sizes we measure span a >1-dex range, from <50 pc to >~500 pc. Uncertainties are based on both the formal fit errors and systematic differences between the public lensing models. These uncertainties range from ~20 pc for the smallest sources to 50 pc for the largest. Using a forward-modeling procedure to model the impact of incompleteness and magnification uncertainties, we characterize the size-luminosity relation at both z~4 and z~6-8. We find that the source sizes of star-forming galaxies at z~4 and z~6-8 scale with luminosity L as L^{0.54\pm0.08} and L^{0.40+/-0.04}, respectively, such that lower luminosity (>~-18 mag) galaxies are smaller than expected from extrapolating the size-luminosity relation at high luminosities (<~-18 mag). The new evidence for a steeper size-luminosity relation (3 sigma) adds to earlier evidence for small sizes based on the prevalence of highly magnified galaxies in high shear regions, theoretical arguments against upturns in the LFs, and other independent determinations of the size-luminosity relation from the HFF clusters., Comment: 22 pages, 18 figures, 2 tables, accepted for publication into ApJ. arXiv admin note: substantial text overlap with arXiv:1711.02090

We compare the sizes and luminosities of faint $z=6$-8 galaxies magnified by the Hubble Frontier Fields (HFF) clusters with star-forming regions, as well as more evolved objects, in the nearby universe. Our high-redshift comparison sample includes 333 z=6-8 galaxies, for which size measurements were made as part of a companion study where lensing magnifications were estimated from various public models. Accurate size measurements for these sources are complicated by the lens model uncertainties, but other results and arguments suggest that faint galaxies are small, as discussed in a companion study. The measured sizes for sources in our comparison sample range from <50 pc to ~500 pc. For many of the lowest luminosity sources, extremely small sizes are inferred, reaching individual sizes as small as 10-30 pc, with several sources in the 10-15 pc range with our conservative magnification limits. The sizes and luminosities are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and -- in a few cases -- super star clusters. The identification of these compact, faint star-forming sources in the z~6-8 universe also allows us to set upper limits on the proto-globular cluster LF at z~6. By comparisons of the counts and sizes with recent models, we rule out (with some caveats) proto-globular cluster formation scenarios favoring substantial (xi=10) post-formation mass loss and set useful upper limits on others. Our size results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift., Comment: 14 pages, 5 figures, AJ, in press. arXiv admin note: substantial text overlap with arXiv:1711.02090

Before the end of the epoch of reionization, the Hydrogen in the Universe was predominantly neutral. This leads to a strong attenuation of Ly$\alpha$ lines of $z\gtrsim6$ galaxies in the intergalactic medium. Nevertheless, Ly$\alpha$ has been detected up to very high redshifts ($z\sim9$) for several especially UV luminous galaxies. Here, we test to what extent the galaxy's local environment might impact the Ly$\alpha$ transmission of such sources. We present an analysis of dedicated Hubble Space Telescope (HST) imaging in the CANDELS/EGS field to search for fainter neighbours around three of the most UV luminous and most distant spectroscopically confirmed Ly$\alpha$ emitters: EGS-zs8-1, EGS-zs8-2 and EGSY-z8p7 at $z_\mathrm{spec}=7.73$, 7.48, and 8.68, respectively. We combine the multi-wavelength HST imaging with Spitzer data to reliably select $z\sim7-9$ galaxies around the central, UV-luminous sources. In all cases, we find a clear enhancement of neighbouring galaxies compared to the expected number in a blank field (by a factor $\sim 3-9\times$). Our analysis thus reveals ubiquitous overdensities around luminous Ly$\alpha$ emitting sources in the heart of the cosmic reionization epoch. We show that our results are in excellent agreement with expectations from the Dragons simulation, confirming the theoretical prediction that the first ionized bubbles preferentially formed in overdense regions. JWST follow-up observations of the neighbouring galaxies identified here will be needed to confirm their physical association and to map out the ionized regions produced by these sources., Comment: 13 pages, 14 figures, Submitted to MNRAS

We constrain the rest-UV size-luminosity relation for star-forming galaxies at z~4 and z~6, 7, and 8 identified behind clusters from the Hubble Frontier Fields (HFF) program. The size-luminosity relation is key to deriving accurate luminosity functions (LF) for faint galaxies. Making use of the latest lensing models and full data set for these clusters, lensing-corrected sizes and luminosities are derived for 68 z~4, 184 z~6, 93 z~7, and 53 z~8 galaxies. We show that size measurements can be reliably measured up to linear magnifications of 30x, where the lensing models are well calibrated. The sizes we measure span a >1-dex range, from <50 pc to >~500 pc. Uncertainties are based on both the formal fit errors and systematic differences between the public lensing models. These uncertainties range from ~20 pc for the smallest sources to 50 pc for the largest. Using a forward-modeling procedure to model the impact of incompleteness and magnification uncertainties, we characterize the size-luminosity relation at both z~4 and z~6-8. We find that the source sizes of star-forming galaxies at z~4 and z~6-8 scale with luminosity L as L^{0.54\pm0.08} and L^{0.40+/-0.04}, respectively, such that lower luminosity (>~-18 mag) galaxies are smaller than expected from extrapolating the size-luminosity relation at high luminosities (<~-18 mag). The new evidence for a steeper size-luminosity relation (3 sigma) adds to earlier evidence for small sizes based on the prevalence of highly magnified galaxies in high shear regions, theoretical arguments against upturns in the LFs, and other independent determinations of the size-luminosity relation from the HFF clusters., Comment: 22 pages, 18 figures, 2 tables, accepted for publication into ApJ. arXiv admin note: substantial text overlap with arXiv:1711.02090

We compare the sizes and luminosities of faint $z=6$-8 galaxies magnified by the Hubble Frontier Fields (HFF) clusters with star-forming regions, as well as more evolved objects, in the nearby universe. Our high-redshift comparison sample includes 333 z=6-8 galaxies, for which size measurements were made as part of a companion study where lensing magnifications were estimated from various public models. Accurate size measurements for these sources are complicated by the lens model uncertainties, but other results and arguments suggest that faint galaxies are small, as discussed in a companion study. The measured sizes for sources in our comparison sample range from <50 pc to ~500 pc. For many of the lowest luminosity sources, extremely small sizes are inferred, reaching individual sizes as small as 10-30 pc, with several sources in the 10-15 pc range with our conservative magnification limits. The sizes and luminosities are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and -- in a few cases -- super star clusters. The identification of these compact, faint star-forming sources in the z~6-8 universe also allows us to set upper limits on the proto-globular cluster LF at z~6. By comparisons of the counts and sizes with recent models, we rule out (with some caveats) proto-globular cluster formation scenarios favoring substantial (xi=10) post-formation mass loss and set useful upper limits on others. Our size results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift., Comment: 14 pages, 5 figures, AJ, in press. arXiv admin note: substantial text overlap with arXiv:1711.02090

Here we provide the most comprehensive determinations of the rest-frame $UV$ LF available to date with HST at z~2, 3, 4, 5, 6, 7, 8, and 9. Essentially all of the non-cluster extragalactic legacy fields are utilized, including the Hubble Ultra Deep Field (HUDF), the Hubble Frontier Field parallel fields, and all five CANDELS fields, for a total survey area of 1136 arcmin^2. Our determinations include galaxies at z~2-3 leveraging the deep HDUV, UVUDF, and ERS WFC3/UVIS observations available over a ~150 arcmin^2 area in the GOODS North and GOODS South regions. All together, our collective samples include >24,000 sources, >2.3x larger than previous selections with HST. 5766, 6332, 7240, 3449, 1066, 601, 246, and 33 sources are identified at z~2, 3, 4, 5, 6, 7, 8, and 9, respectively. Combining our results with an earlier z~10 LF determination by Oesch+2018a, we quantify the evolution of the $UV$ LF. Our results indicate that there is (1) a smooth flattening of the faint-end slope alpha from alpha~-2.4 at z~10 to -1.5 at z~2, (2) minimal evolution in the characteristic luminosity M* at z>~2.5, and (3) a monotonic increase in the normalization log_10 phi* from z~10 to z~2, which can be well described by a simple second-order polynomial, consistent with an "accelerated" evolution scenario. We find that each of these trends (from z~10 to z~2.5 at least) can be readily explained on the basis of the evolution of the halo mass function and a simple constant star formation efficiency model., Comment: 18 pages, 10 figures, 5 tables, in submission to ApJ, figures 9 and 10 show the main result

Here we provide the most comprehensive determinations of the rest-frame UV luminosity function (LF) available to date with the Hubble Space Telescope (HST) at z similar to 2-9. Essentially all of the noncluster extragalactic legacy fields are utilized, including the Hubble Ultra Deep Field, the Hubble Frontier Fields parallel fields, and all five CANDELS fields, for a total survey area of 1136 arcmin(2). Our determinations include galaxies at z similar to 2-3 leveraging the deep HDUV, UVUDF, and ERS WFC3/UVIS observations available over an similar to 150 arcmin(2) area in the GOODS-North and GOODS-South regions. All together, our collective samples include >24,000 sources, >2.3x larger than previous selections with HST. We identify 5766, 6332, 7240, 3449, 1066, 601, 246, and 33 sources at z similar to 2, 3, 4, 5, 6, 7, 8, and 9, respectively. Combining our results with an earlier z similar to 10 LF determination by Oesch et al., we quantify the evolution of the UV LF. Our results indicate that there is (1) a smooth flattening of the faint-end slope alpha from alpha similar to -2.4 at z similar to 10 to alpha similar to -1.5 at z similar to 2, (2) minimal evolution in the characteristic luminosity M* at z >= 2.5, and (3) a monotonic increase in the normalization log(10) phi* from z similar to 10 to 2, which can be well described by a simple second-order polynomial, consistent with an "accelerated" evolution scenario. We find that each of these trends (from z similar to 10 to 2.5 at least) can be readily explained on the basis of the evolution of the halo mass function and a simple constant star formation efficiency model.

We report the results of an expanded search for z~9-10 candidates over the ~883 arcmin^2 CANDELS+ERS fields. This study adds 147 arcmin^2 to the search area we consider over the CANDELS COSMOS, UDS, and EGS fields, while expanding our selection to include sources with bluer J_{125}-H_{160} colors than our previous J_{125}-H_{160}>0.5 mag selection. In searching for new z~9-10 candidates, we make full use of all available HST, Spitzer/IRAC, and ground-based imaging data. As a result of our expanded search and use of broader color criteria, 3 new candidate z~9-10 galaxies are identified. We also find again the z=8.683 source previously confirmed by Zitrin+2015. This brings our sample of probable z~9-11 galaxy candidates over the CANDELS+ERS fields to 19 sources in total, equivalent to 1 candidate per 47 arcmin^2 (1 per 10 WFC3/IR fields). To be comprehensive, we also discuss 28 mostly lower likelihood z~9-10 candidates, including some sources that seem to be reliably at z>8 using the HST+IRAC data alone, but which the ground-based data show are much more likely at z<4. One case example is a bright z~9.4 candidate COS910-8 which seems instead to be at z~2. Based on this expanded sample, we obtain a more robust LF at z~9 and improved constraints on the volume density of bright z~9 and z~10 galaxies. Our improved z~9-10 results again reinforce previous findings for strong evolution in the UV LF at z>8, with a factor of ~10 evolution seen in the luminosity density from z~10 to z~8., Comment: 22 pages, 12 figures, 6 tables, accepted for publication in the Astrophysical Journal

The James Webb Space Telescope will allow to spectroscopically study an unprecedented number of galaxies deep into the reionization era, notably by detecting [OIII] and H$\beta$ nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at $z\sim8$ and study their rest-frame optical emission lines. Combining data from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) survey and from HST, we perform spectral energy distribution (SED) fitting, using synthetic SEDs from a large grid of photoionization models. The deep Spitzer/IRAC data combined with our models exploring a large parameter space enables to constrain the [OIII]+H$\beta$ fluxes and equivalent widths for our sample, as well as the average physical properties of $z\sim8$ galaxies, such as the ionizing photon production efficiency with $\log(\xi_\mathrm{ion}/\mathrm{erg}^{-1}\hspace{1mm}\mathrm{Hz})\geq25.77$. We find a relatively tight correlation between the [OIII]+H$\beta$ and UV luminosity, which we use to derive for the first time the [OIII]+H$\beta$ luminosity function (LF) at $z\sim8$. The $z\sim8$ [OIII]+H$\beta$ LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [OIII]+H$\beta$ luminosity at a given UV luminosity from $z\sim3$ to $z\sim8$. Finally, using the [OIII]+H$\beta$ LF, we make predictions for JWST/NIRSpec number counts of $z\sim8$ galaxies. We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for $z\sim8$ spectroscopy even at 1hr depth and JWST pre-imaging to $\gtrsim30$ mag will be required., Comment: 13 pages, 9 figures, accepted for publication in MNRAS

Studying the formation and evolution of galaxies at the earliest cosmic times, and their role in reionization, requires the deepest imaging possible. Ultra-deep surveys like the HUDF and HFF have pushed to mag \mAB$\,\sim\,$30, revealing galaxies at the faint end of the LF to $z$$\,\sim\,$9$\,-\,$11 and constraining their role in reionization. However, a key limitation of these fields is their size, only a few arcminutes (less than a Mpc at these redshifts), too small to probe large-scale environments or clustering properties of these galaxies, crucial for advancing our understanding of reionization. Achieving HUDF-quality depth over areas $\sim$100 times larger becomes possible with a mission like the Wide Field Infrared Survey Telescope (WFIRST), a 2.4-m telescope with similar optical properties to HST, with a field of view of $\sim$1000 arcmin$^2$, $\sim$100$\times$ the area of the HST/ACS HUDF. This whitepaper motivates an Ultra-Deep Field survey with WFIRST, covering $\sim$100$\,-\,$300$\times$ the area of the HUDF, or up to $\sim$1 deg$^2$, to \mAB$\,\sim\,$30, potentially revealing thousands of galaxies and AGN at the faint end of the LF, at or beyond $z$\,$\sim$\,9$\,-\,$10 in the epoch of reionization, and tracing their LSS environments, dramatically increasing the discovery potential at these redshifts. (Note: This paper is a somewhat expanded version of one that was submitted as input to the Astro2020 Decadal Survey, with this version including an Appendix (which exceeded the Astro2020 page limits), describing how the science drivers for a WFIRST Ultra Deep Field might map into a notional observing program, including the filters used and exposure times needed to achieve these depths.)

Studying the formation and evolution of galaxies at the earliest cosmic times, and their role in reionization, requires the deepest imaging possible. Ultra-deep surveys like the HUDF and HFF have pushed to mag m_(AB)∼30, revealing galaxies at the faint end of the LF to z∼9−11 and constraining their role in reionization. However, a key limitation of these fields is their size, only a few arcminutes (less than a Mpc at these redshifts), too small to probe large-scale environments or clustering properties of these galaxies, crucial for advancing our understanding of reionization. Achieving HUDF-quality depth over areas ∼100 times larger becomes possible with a mission like the Wide Field Infrared Survey Telescope (WFIRST), a 2.4-m telescope with similar optical properties to HST, with a field of view of ∼1000 arcmin^2, ∼100× the area of the HST/ACS HUDF. This whitepaper motivates an Ultra-Deep Field survey with WFIRST, covering ∼100−300× the area of the HUDF, or up to ∼1 deg^2, to m_(AB)∼30, potentially revealing thousands of galaxies and AGN at the faint end of the LF, at or beyond z∼9−10 in the epoch of reionization, and tracing their LSS environments, dramatically increasing the discovery potential at these redshifts. (Note: This paper is a somewhat expanded version of one that was submitted as input to the Astro2020 Decadal Survey, with this version including an Appendix (which exceeded the Astro2020 page limits), describing how the science drivers for a WFIRST Ultra Deep Field might map into a notional observing program, including the filters used and exposure times needed to achieve these depths.)

We report the results of an expanded search for z~9-10 candidates over the ~883 arcmin^2 CANDELS+ERS fields. This study adds 147 arcmin^2 to the search area we consider over the CANDELS COSMOS, UDS, and EGS fields, while expanding our selection to include sources with bluer J_{125}-H_{160} colors than our previous J_{125}-H_{160}>0.5 mag selection. In searching for new z~9-10 candidates, we make full use of all available HST, Spitzer/IRAC, and ground-based imaging data. As a result of our expanded search and use of broader color criteria, 3 new candidate z~9-10 galaxies are identified. We also find again the z=8.683 source previously confirmed by Zitrin+2015. This brings our sample of probable z~9-11 galaxy candidates over the CANDELS+ERS fields to 19 sources in total, equivalent to 1 candidate per 47 arcmin^2 (1 per 10 WFC3/IR fields). To be comprehensive, we also discuss 28 mostly lower likelihood z~9-10 candidates, including some sources that seem to be reliably at z>8 using the HST+IRAC data alone, but which the ground-based data show are much more likely at z<4. One case example is a bright z~9.4 candidate COS910-8 which seems instead to be at z~2. Based on this expanded sample, we obtain a more robust LF at z~9 and improved constraints on the volume density of bright z~9 and z~10 galaxies. Our improved z~9-10 results again reinforce previous findings for strong evolution in the UV LF at z>8, with a factor of ~10 evolution seen in the luminosity density from z~10 to z~8., Comment: 22 pages, 12 figures, 6 tables, accepted for publication in the Astrophysical Journal

The James Webb Space Telescope will allow to spectroscopically study an unprecedented number of galaxies deep into the reionization era, notably by detecting [OIII] and H$\beta$ nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at $z\sim8$ and study their rest-frame optical emission lines. Combining data from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) survey and from HST, we perform spectral energy distribution (SED) fitting, using synthetic SEDs from a large grid of photoionization models. The deep Spitzer/IRAC data combined with our models exploring a large parameter space enables to constrain the [OIII]+H$\beta$ fluxes and equivalent widths for our sample, as well as the average physical properties of $z\sim8$ galaxies, such as the ionizing photon production efficiency with $\log(\xi_\mathrm{ion}/\mathrm{erg}^{-1}\hspace{1mm}\mathrm{Hz})\geq25.77$. We find a relatively tight correlation between the [OIII]+H$\beta$ and UV luminosity, which we use to derive for the first time the [OIII]+H$\beta$ luminosity function (LF) at $z\sim8$. The $z\sim8$ [OIII]+H$\beta$ LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [OIII]+H$\beta$ luminosity at a given UV luminosity from $z\sim3$ to $z\sim8$. Finally, using the [OIII]+H$\beta$ LF, we make predictions for JWST/NIRSpec number counts of $z\sim8$ galaxies. We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for $z\sim8$ spectroscopy even at 1hr depth and JWST pre-imaging to $\gtrsim30$ mag will be required., Comment: 13 pages, 9 figures, accepted for publication in MNRAS

Studying the formation and evolution of galaxies at the earliest cosmic times, and their role in reionization, requires the deepest imaging possible. Ultra-deep surveys like the HUDF and HFF have pushed to mag \mAB$\,\sim\,$30, revealing galaxies at the faint end of the LF to $z$$\,\sim\,$9$\,-\,$11 and constraining their role in reionization. However, a key limitation of these fields is their size, only a few arcminutes (less than a Mpc at these redshifts), too small to probe large-scale environments or clustering properties of these galaxies, crucial for advancing our understanding of reionization. Achieving HUDF-quality depth over areas $\sim$100 times larger becomes possible with a mission like the Wide Field Infrared Survey Telescope (WFIRST), a 2.4-m telescope with similar optical properties to HST, with a field of view of $\sim$1000 arcmin$^2$, $\sim$100$\times$ the area of the HST/ACS HUDF. This whitepaper motivates an Ultra-Deep Field survey with WFIRST, covering $\sim$100$\,-\,$300$\times$ the area of the HUDF, or up to $\sim$1 deg$^2$, to \mAB$\,\sim\,$30, potentially revealing thousands of galaxies and AGN at the faint end of the LF, at or beyond $z$\,$\sim$\,9$\,-\,$10 in the epoch of reionization, and tracing their LSS environments, dramatically increasing the discovery potential at these redshifts. (Note: This paper is a somewhat expanded version of one that was submitted as input to the Astro2020 Decadal Survey, with this version including an Appendix (which exceeded the Astro2020 page limits), describing how the science drivers for a WFIRST Ultra Deep Field might map into a notional observing program, including the filters used and exposure times needed to achieve these depths.)

We present the Hubble Deep UV Legacy Survey (HDUV), a 132 orbit imaging program with the WFC3/UVIS camera onboard the Hubble Space Telescope (HST). The HDUV extends and builds on the few previous UV imaging surveys in the two GOODS/CANDELS-Deep fields to provide deep images over a total area of ~100 arcmin2 in the two filters F275W and F336W. Our release also includes all the F275W imaging data taken by the CANDELS survey, which were aligned using a novel approach and combined with the HDUV survey data. By reaching depths of 27.5-28.0 mag (5sigma, in 0.4" apertures), these are the deepest high-resolution UV data over such a large area taken to date. Such unique UV imaging enables a wide range of science by the community. Among the main goals of the HDUV survey are: (1) provide a complete sample of faint star-forming galaxies at z~1-3, (2) constrain the ionizing photon escape fraction from galaxies at z~2-3, and (3) track the build-up of bulges and the disappearance of clumpy disk galaxies through reliable internal stellar population properties at sub-kpc resolution out to z~3. The addition of the HDUV data further enhances the legacy value of the two GOODS/CANDELS-Deep fields, which now include deep 11-band HST imaging as well as very deep ancillary data from X-ray to radio, enabling unique multi-wavelength studies. Here, we provide an overview of the survey design, describe the data reduction, and highlight a few basic analyses on the images which are released to the community as high level science products via the Mikulski Archive for Space Telescopes (MAST)., Comment: 11 pages, 10 figures, accepted for publication in ApJS

We present the Hubble Deep UV Legacy Survey (HDUV), a 132 orbit imaging program with the WFC3/UVIS camera onboard the Hubble Space Telescope (HST). The HDUV extends and builds on the few previous UV imaging surveys in the two GOODS/CANDELS-Deep fields to provide deep images over a total area of ~100 arcmin2 in the two filters F275W and F336W. Our release also includes all the F275W imaging data taken by the CANDELS survey, which were aligned using a novel approach and combined with the HDUV survey data. By reaching depths of 27.5-28.0 mag (5sigma, in 0.4" apertures), these are the deepest high-resolution UV data over such a large area taken to date. Such unique UV imaging enables a wide range of science by the community. Among the main goals of the HDUV survey are: (1) provide a complete sample of faint star-forming galaxies at z~1-3, (2) constrain the ionizing photon escape fraction from galaxies at z~2-3, and (3) track the build-up of bulges and the disappearance of clumpy disk galaxies through reliable internal stellar population properties at sub-kpc resolution out to z~3. The addition of the HDUV data further enhances the legacy value of the two GOODS/CANDELS-Deep fields, which now include deep 11-band HST imaging as well as very deep ancillary data from X-ray to radio, enabling unique multi-wavelength studies. Here, we provide an overview of the survey design, describe the data reduction, and highlight a few basic analyses on the images which are released to the community as high level science products via the Mikulski Archive for Space Telescopes (MAST)., Comment: 11 pages, 10 figures, accepted for publication in ApJS

We present an analysis of all prime HST legacy fields spanning >800 arcmin^2 for the search of z~10 galaxy candidates and the study of their UV luminosity function (LF). In particular, we present new z~10 candidates selected from the full Hubble Frontier Field (HFF) dataset. Despite the addition of these new fields, we find a low abundance of z~10 candidates with only 9 reliable sources identified in all prime HST datasets that include the HUDF09/12, the HUDF/XDF, all the CANDELS fields, and now the HFF survey. Based on this comprehensive search, we find that the UV luminosity function decreases by one order of magnitude from z~8 to z~10 at all luminosities over a four magnitude range. This also implies a decrease of the cosmic star-formation rate density by an order of magnitude within 170 Myr from z~8 to z~10. We show that this accelerated evolution compared to lower redshift can entirely be explained by the fast build-up of the dark matter halo mass function at z>8. Consequently, the predicted UV LFs from several models of galaxy formation are in good agreement with this observed trend, even though the measured UV LF lies at the low end of model predictions. In particular, the number of only 9 observed candidate galaxies is lower, by ~50%, than predicted by galaxy evolution models. The difference is generally still consistent within the Poisson and cosmic variance uncertainties. However, essentially all models predict larger numbers than observed. We discuss the implications of these results in light of the upcoming James Webb Space Telescope mission, which is poised to find much larger samples of z~10 galaxies as well as their progenitors at less than 400 Myr after the Big Bang., Comment: 13 pages, 6 figures, minor updates to match accepted version

We compare the sizes and luminosities of 307 faint z=6-8 sources revealed by the Hubble Frontier Fields (HFF) program with sources in the nearby universe. Making use of the latest lensing models and data from the first four HFF clusters with an extensive suite of public lens models, we measure both the sizes and luminosities for 153 z~6, 101 z~7, and 53 z~8 galaxies. The sizes range over more than a decade from ~500 to <50 pc. Extremely small sizes are inferred for many of our lowest luminosity sources, reaching individual sizes as small as 10-30 pc (the smallest is 11(-6)(+28) pc). The uncertainty in these measures ranges from 80 pc for the largest sources to typically about 20 pc for the smallest. Such sizes are smaller than extrapolations of the size-luminosity relation, and expectations for the completeness of our faint samples, suggesting a likely break in the size-luminosity relation at ~-17 mag with size proportional to L**(0.50(-0.11)(+0.10)). The sizes and luminosities of the lowest-luminosity sources are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and -- in a few cases -- super star clusters. Remarkably, our identification of these compact, faint star-forming sources in the z~6-8 universe also allow us to set upper limits on the proto-globular cluster LF at z~6. Comparisons with recent models allow us to rule out (with some caveats) some scenarios for proto-globular cluster formation and set useful upper limits on other less extreme ones. Our results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift., Comment: The authors have decided to withdraw this manuscript from the Astrophysical Journal given the unprofessional footnotes in the original astro-ph posting, offering our sincere apologies

We compare the sizes and luminosities of 307 faint z=6-8 sources revealed by the Hubble Frontier Fields (HFF) program with sources in the nearby universe. Making use of the latest lensing models and data from the first four HFF clusters with an extensive suite of public lens models, we measure both the sizes and luminosities for 153 z~6, 101 z~7, and 53 z~8 galaxies. The sizes range over more than a decade from ~500 to <50 pc. Extremely small sizes are inferred for many of our lowest luminosity sources, reaching individual sizes as small as 10-30 pc (the smallest is 11(-6)(+28) pc). The uncertainty in these measures ranges from 80 pc for the largest sources to typically about 20 pc for the smallest. Such sizes are smaller than extrapolations of the size-luminosity relation, and expectations for the completeness of our faint samples, suggesting a likely break in the size-luminosity relation at ~-17 mag with size proportional to L**(0.50(-0.11)(+0.10)). The sizes and luminosities of the lowest-luminosity sources are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and -- in a few cases -- super star clusters. Remarkably, our identification of these compact, faint star-forming sources in the z~6-8 universe also allow us to set upper limits on the proto-globular cluster LF at z~6. Comparisons with recent models allow us to rule out (with some caveats) some scenarios for proto-globular cluster formation and set useful upper limits on other less extreme ones. Our results suggest we may be very close to discovering a bona-fide population of forming globular clusters at high redshift., Comment: The authors have decided to withdraw this manuscript from the Astrophysical Journal given the unprofessional footnotes in the original astro-ph posting, offering our sincere apologies

We present an analysis of all prime HST legacy fields spanning >800 arcmin^2 for the search of z~10 galaxy candidates and the study of their UV luminosity function (LF). In particular, we present new z~10 candidates selected from the full Hubble Frontier Field (HFF) dataset. Despite the addition of these new fields, we find a low abundance of z~10 candidates with only 9 reliable sources identified in all prime HST datasets that include the HUDF09/12, the HUDF/XDF, all the CANDELS fields, and now the HFF survey. Based on this comprehensive search, we find that the UV luminosity function decreases by one order of magnitude from z~8 to z~10 at all luminosities over a four magnitude range. This also implies a decrease of the cosmic star-formation rate density by an order of magnitude within 170 Myr from z~8 to z~10. We show that this accelerated evolution compared to lower redshift can entirely be explained by the fast build-up of the dark matter halo mass function at z>8. Consequently, the predicted UV LFs from several models of galaxy formation are in good agreement with this observed trend, even though the measured UV LF lies at the low end of model predictions. In particular, the number of only 9 observed candidate galaxies is lower, by ~50%, than predicted by galaxy evolution models. The difference is generally still consistent within the Poisson and cosmic variance uncertainties. However, essentially all models predict larger numbers than observed. We discuss the implications of these results in light of the upcoming James Webb Space Telescope mission, which is poised to find much larger samples of z~10 galaxies as well as their progenitors at less than 400 Myr after the Big Bang., Comment: 13 pages, 6 figures, minor updates to match accepted version

We present six galaxies at z~2 that show evidence of Lyman continuum (LyC) emission based on the newly acquired UV imaging of the Hubble Deep UV legacy survey (HDUV) conducted with the WFC3/UVIS camera on the Hubble Space Telescope (HST). At the redshift of these sources, the HDUV F275W images partially probe the ionizing continuum. By exploiting the HST multi-wavelength data available in the HDUV/GOODS fields, models of the UV spectral energy distributions, and detailed Monte Carlo simulations of the intergalactic medium absorption, we estimate the absolute ionizing photon escape fractions of these galaxies to be very high -- typically >60% (>13% for all sources at 90% likelihood). Our findings are in broad agreement with previous studies that found only a small fraction of galaxies to show high escape fraction. These six galaxies comprise the largest sample yet of LyC leaking candidates at z~2 whose inferred LyC flux has been cleanly observed at HST resolution. While three of our six candidates show evidence of hosting an active galactic nucleus (AGN), two of these are heavily obscured and their LyC emission appears to originate from star-forming regions rather than the central nucleus. This suggests an AGN-aided pathway for LyC escape from these sources. Extensive multi-wavelength data in the GOODS fields, especially the near-IR grism spectra from the 3D-HST survey, enable us to study the candidates in detail and tentatively test some recently proposed indirect methods to probe LyC leakage -- namely, the [OIII]/[OII] line ratio and the H$\beta-$UV slope diagram. High-resolution spectroscopic followup of our candidates will help constrain such indirect methods which are our only hope of studying $f_{esc}$ at z~5-9 in the fast-approaching era of the James Webb Space Telescope., Comment: 11 pages, 6 figures, submitted to ApJ

We use the largest sample of z~6 galaxies to date from the first four Hubble Frontier Fields clusters to set constraints on the shape of the z~6 luminosity functions (LFs) to fainter than Muv=-14 mag. We quantify, for the first time, the impact of magnification uncertainties on LF results and thus provide more realistic constraints than other recent work. Our simulations reveal that for the highly-magnified sources the systematic uncertainties can become extremely large fainter than -14 mag, reaching several orders of magnitude at 95% confidence at ~-12 mag. Our new forward-modeling formalism incorporates the impact of magnification uncertainties into the LF results by exploiting the availability of many independent magnification models for the same cluster. One public magnification model is used to construct a mock high-redshift galaxy sample that is then analyzed using the other magnification models to construct a LF. Large systematic errors occur at high magnifications (mu>30) because of differences between the models. The volume densities we derive for faint (>-17 mag) sources are ~3-4x lower than one recent report and give a faint-end slope alpha=-1.92+/-0.04, which is 3.0-3.5sigma shallower (including or not including the size uncertainties, respectively). We introduce a new curvature parameter delta to model the faint end of the LF and demonstrate that the observations permit (at 68% confidence) a turn-over at z~6 in the range -15.3 to -14.2 mag, depending on the assumed lensing model. The present consideration of magnification errors and new size determinations raise doubts about previous reports regarding the form of the LF at >-14 mag. We discuss the implications of our turn-over constraints in the context of recent theoretical predictions., Comment: 33 pages, 19 figures, 5 tables, accepted for publication in the Astrophysical Journal, updated to match the version in press

We provide the first observational constraints on the sizes of the faintest galaxies lensed by the Hubble Frontier Fields (HFF) clusters. Ionizing radiation from faint galaxies likely drives cosmic reionization, and the HFF initiative provides a key opportunity to find such galaxies. Yet, we cannot really assess their ionizing emissivity without a robust measurement of their sizes, since this is key to quantifying both their prevalence and the faint-end slope to the UV luminosity function. Here we provide the first such size constraints with 2 new techniques. The first utilizes the fact that the detectability of highly-magnified galaxies as a function of shear is very dependent on a galaxy's size. Only the most compact galaxies will remain detectable in regions of high shear (vs. a larger detectable size range for low shear), a phenomenon we carefully quantify using simulations. Remarkably, however, no correlation is found between the surface density of faint galaxies and the predicted shear, using 87 faint high-magnification mu>10 z~2-8 galaxies seen behind the first 4 HFF clusters. This can only be the case if such faint (~-15 mag) galaxies have significantly smaller sizes than luminous galaxies. We constrain their half-light radii to be <~30 mas (<160-240 pc). As a 2nd size probe, we rotate and stack 26 faint high-magnification sources along the major shear axis. Less elongation is found than even for objects with an intrinsic half-light radius of 10 mas. Together these results indicate that extremely faint z~2-8 galaxies have near point-source profiles in the HFF dataset (half-light radii conservatively <30 mas and likely 5-10 mas). These results suggest smaller completeness corrections and hence much lower volume densities for faint z~2-8 galaxies and shallower faint-end slopes than have been derived in many recent studies (by factors of ~2-3 and by dalpha>~0.1-0.3)., Comment: 19 pages, 15 figures, 3 tables, accepted for publication in ApJ

We present Hubble WFC3/IR slitless grism spectra of a remarkably bright $z\gtrsim10$ galaxy candidate, GN-z11, identified initially from CANDELS/GOODS-N imaging data. A significant spectroscopic continuum break is detected at $\lambda=1.47\pm0.01~\mu$m. The new grism data, combined with the photometric data, rule out all plausible lower redshift solutions for this source. The only viable solution is that this continuum break is the Ly$\alpha$ break redshifted to ${z_\mathrm{grism}=11.09^{+0.08}_{-0.12}}$, just $\sim$400 Myr after the Big Bang. This observation extends the current spectroscopic frontier by 150 Myr to well before the Planck (instantaneous) cosmic reionization peak at z~8.8, demonstrating that galaxy build-up was well underway early in the reionization epoch at z>10. GN-z11 is remarkably and unexpectedly luminous for a galaxy at such an early time: its UV luminosity is 3x larger than L* measured at z~6-8. The Spitzer IRAC detections up to 4.5 $\mu$m of this galaxy are consistent with a stellar mass of ${\sim10^{9}~M_\odot}$. This spectroscopic redshift measurement suggests that the James Webb Space Telescope (JWST) will be able to similarly and easily confirm such sources at z>10 and characterize their physical properties through detailed spectroscopy. Furthermore, WFIRST, with its wide-field near-IR imaging, would find large numbers of similar galaxies and contribute greatly to JWST's spectroscopy, if it is launched early enough to overlap with JWST., Comment: 12 pages, 11 figures, accepted for publication in ApJ

We present six galaxies at z~2 that show evidence of Lyman continuum (LyC) emission based on the newly acquired UV imaging of the Hubble Deep UV legacy survey (HDUV) conducted with the WFC3/UVIS camera on the Hubble Space Telescope (HST). At the redshift of these sources, the HDUV F275W images partially probe the ionizing continuum. By exploiting the HST multi-wavelength data available in the HDUV/GOODS fields, models of the UV spectral energy distributions, and detailed Monte Carlo simulations of the intergalactic medium absorption, we estimate the absolute ionizing photon escape fractions of these galaxies to be very high -- typically >60% (>13% for all sources at 90% likelihood). Our findings are in broad agreement with previous studies that found only a small fraction of galaxies to show high escape fraction. These six galaxies comprise the largest sample yet of LyC leaking candidates at z~2 whose inferred LyC flux has been cleanly observed at HST resolution. While three of our six candidates show evidence of hosting an active galactic nucleus (AGN), two of these are heavily obscured and their LyC emission appears to originate from star-forming regions rather than the central nucleus. This suggests an AGN-aided pathway for LyC escape from these sources. Extensive multi-wavelength data in the GOODS fields, especially the near-IR grism spectra from the 3D-HST survey, enable us to study the candidates in detail and tentatively test some recently proposed indirect methods to probe LyC leakage -- namely, the [OIII]/[OII] line ratio and the H$\beta-$UV slope diagram. High-resolution spectroscopic followup of our candidates will help constrain such indirect methods which are our only hope of studying $f_{esc}$ at z~5-9 in the fast-approaching era of the James Webb Space Telescope., Comment: 11 pages, 6 figures, submitted to ApJ