05/2019
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.
more3/2018
We present an analysis of all prime HST legacy fields spanning >800 arcmin2 in the search for 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) data set. Despite the addition of these new fields, we find a low abundance of z ∼ 10 candidates with only nine reliable sources identified in all prime HST data sets that include the HUDF09/12, the HUDF/XDF, all of 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 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. The difference is generally still consistent within the Poisson and cosmic variance uncertainties. 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. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.
more1/2018
The earliest galaxies are thought to have emerged during the first billion years of cosmic history, initiating the ionization of the neutral hydrogen that pervaded the Universe at this time. Studying this ‘epoch of reionization’ involves looking for the spectral signatures of ancient galaxies that are, owing to the expansion of the Universe, now very distant from Earth and therefore exhibit large redshifts. However, finding these spectral fingerprints is challenging. One spectral characteristic of ancient and distant galaxies is strong hydrogen-emission lines (known as Lyman-α lines), but the neutral intergalactic medium that was present early in the epoch of reionization scatters such Lyman-α photons. Another potential spectral identifier is the line at wavelength 157.4 micrometres of the singly ionized state of carbon (the [C II] λ = 157.74 μm line), which signifies cooling gas and is expected to have been bright in the early Universe. However, so far Lyman-α-emitting galaxies from the epoch of reionization have demonstrated much fainter [C II] luminosities than would be expected from local scaling relations, and searches for the [C II] line in sources without Lyman-α emission but with photometric redshifts greater than 6 (corresponding to the first billion years of the Universe) have been unsuccessful. Here we identify [C II] λ = 157.74 μm emission from two sources that we selected as high-redshift candidates on the basis of near-infrared photometry; we confirm that these sources are two galaxies at redshifts of z = 6.8540 ± 0.0003 and z = 6.8076 ± 0.0002. Notably, the luminosity of the [C II] line from these galaxies is higher than that found previously in star-forming galaxies with redshifts greater than 6.5. The luminous and extended [C II] lines reveal clear velocity gradients that, if interpreted as rotation, would indicate that these galaxies have similar dynamic properties to the turbulent yet rotation-dominated disks that have been observed in Hα-emitting galaxies two billion years later, at ‘cosmic noon’.
more1/2018
We use a newly assembled sample of 3545 star-forming galaxies with secure spectroscopic, grism, and photometric redshifts at z = 1.5–2.5 to constrain the relationship between UV slope (β) and dust attenuation (L IR/L UV ≡ IRX). Our sample significantly extends the range of L UV and β probed in previous UV-selected samples, including those as faint as M 1600 = −17.4 (≃ 0.05{L}{UV}* ) and −2.6 ≲ β ≲ 0.0. IRX is measured using stacks of deep Herschel data, and the results are compared with predictions of the IRX−β relation for different assumptions of the stellar population model and obscuration curve. We find that z = 1.5–2.5 galaxies have an IRX−β relation that is consistent with the predictions for an SMC curve if we invoke subsolar-metallicity models currently favored for high-redshift galaxies, while the commonly assumed starburst curve overpredicts the IRX at a given β by a factor of ≳3. IRX is roughly constant with L UV for L UV ≳ 3 × 109 L &sun;. Thus, the commonly observed trend of fainter galaxies having bluer β may simply reflect bluer intrinsic slopes for such galaxies, rather than lower obscurations. The IRX−β relation for young/low-mass galaxies at z ≳ 2 implies a dust curve that is steeper than the SMC. The lower attenuations and higher ionizing photon output for low-metallicity stellar populations point to Lyman continuum production efficiencies, ξ ion, that may be elevated by a factor of ≈2 relative to the canonical value for L* galaxies, aiding in their ability to keep the universe ionized at z ∼ 2.
more11/2017
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.
more9/2017
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 multiwavelength 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 with high escape fraction. These six galaxies compose the largest sample yet of LyC leaking candidates at z˜ 2 whose inferred LyC flux has been observed at HST resolution. While three of our six candidates show evidence of hosting an active galactic nucleus, two of these are heavily obscured and their LyC emission appears to originate from star-forming regions rather than the central nucleus. Extensive multiwavelength 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. High-resolution spectroscopic follow-up of our candidates will help constrain such indirect methods, which are our only hope of studying f esc at z˜ 5-9 in the JWST era. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
more7/2017
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 {M}{UV,{AB}}=-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 approximately −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 an LF. Large systematic errors occur at high magnifications (μ ≳ 30) because of differences between the models. The volume densities we derive for faint (≳−17 mag) sources are ∼3–4× lower than one recent report and give a faint-end slope α =-1.92+/- 0.04, which is 3.0–3.5σ shallower (including or not including the size uncertainties, respectively). We introduce a new curvature parameter δ 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 of −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.
more7/2017
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. However, we cannot 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 size constraints with two 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 remain detectable in high-shear regions (versus a larger detectable size range for low shear), a phenomenon we quantify using simulations. Remarkably, however, no correlation is found between the surface density of faint galaxies and the predicted shear, using 87 high-magnification (μ =10–100) z∼ 2–8 galaxies seen behind the first four HFF clusters. This can only be the case if faint (∼ -15 mag) galaxies have significantly smaller sizes than more luminous galaxies, i.e., ≲ 30 mas or 160–240 pc. As a second size probe, we rotate and stack 26 faint high-magnification sources along the major shear axis. Less elongation is found 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 (half-light radii <30 mas and perhaps 5–10 mas). These results suggest smaller completeness corrections and hence shallower faint-end slopes for the z∼ 2–8 LFs than derived in some recent studies (by {{Δ }}α ≳ 0.1–0.3).
more10/2016
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.
more10/2016
The deep, wide-area (˜800-900 arcmin2) near-infrared/WFC3/IR + Spitzer/IRAC observations over the CANDELS fields have been a remarkable resource for constraining the bright end of high-redshift UV luminosity functions. However, the lack of Hubble Space Telescope (HST) 1.05 μm observations over the CANDELS fields has made it difficult to identify z ˜ 9-10 sources robustly, since such data are needed to confirm the presence of an abrupt Lyman break at 1.2 μm. Here, we report on the successful identification of many such z ˜ 9-10 sources from a new HST program (z9-CANDELS) that targets the highest-probability z ˜ 9-10 galaxy candidates with observations at 1.05 μm, to search for a robust Lyman-break at 1.2 μm. The potential z ˜ 9-10 candidates were preselected from the full HST, Spitzer/IRAC S-CANDELS observations, and the deepest-available ground-based optical+near-infrared observations (CFHTLS-DEEP+HUGS+UltraVISTA+ZFOURGE). We identified 15 credible z ˜ 9-10 galaxies over the CANDELS fields. Nine of these galaxies lie at z ˜ 9 and five are new identifications. Our targeted follow-up strategy has proven to be very efficient in making use of scarce HST time to secure a reliable sample of z ˜ 9-10 galaxies. Through extensive simulations, we replicate the selection process for our sample (both the preselection and follow-up) and use it to improve current estimates for the volume density of bright z ˜ 9 and z ˜ 10 galaxies. The volume densities we find are 5{}-2+3× and {8}-3+9× lower, respectively, than those found at z ˜ 8. When compared with the best-fit evolution (i.e., d {{log}}10 {ρ }{UV}/{dz}=-0.29+/- 0.02) in the UV luminosity densities from z ˜ 8 to z ˜ 4 integrated to 0.3{L}z=3* (-20 mag), these luminosity densities are {2.6}-0.9+1.5× and {2.2}-1.1+2.0× lower, respectively, than the extrapolated trends. Our new results are broadly consistent with the “accelerated evolution” scenario at z > 8, consistent with that seen in many models. 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.
more8/2016
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).
more6/2016
We identify four unusually bright (H {}160,{AB} < 25.5) galaxies from Hubble Space Telescope (HST) and Spitzer CANDELS data with probable redshifts z ˜ 7-9. These identifications include the brightest-known galaxies to date at z ≳ 7.5. As Y-band observations are not available over the full CANDELS program to perform a standard Lyman-break selection of z > 7 galaxies, we employ an alternate strategy using deep Spitzer/IRAC data. We identify z ˜ 7.1-9.1 galaxies by selecting z ≳ 6 galaxies from the HST CANDELS data that show quite red IRAC [3.6]-[4.5] colors, indicating strong [O iii]+Hβ lines in the 4.5 μm band. This selection strategy was validated using a modest sample for which we have deep Y-band coverage, and subsequently used to select the brightest z ≥ 7 sources. Applying the IRAC criteria to all HST-selected optical dropout galaxies over the full ˜900 arcmin2 of the CANDELS survey revealed four unusually bright z ˜ 7.1, 7.6, 7.9, and 8.6 candidates. The median [3.6]-[4.5] color of our selected z ˜ 7.1-9.1 sample is consistent with rest-frame [O iii]+Hβ EWs of ˜1500 Å in the [4.5] band. Keck/MOSFIRE spectroscopy has been independently reported for two of our selected sources, showing Lyα at redshifts of 7.7302 ± 0.0006 and {8.683}-0.004+0.001, respectively. We present similar Keck/MOSFIRE spectroscopy for a third selected galaxy with a probable 4.7σ Lyα line at z spec = 7.4770 ± 0.0008. All three have H160-band magnitudes of ˜25 mag and are ˜0.5 mag more luminous (M 1600 ˜ -22.0) than any previously discovered z ˜ 8 galaxy, with important implications for the UV luminosity function (LF). Our three brightest and highest redshift z > 7 galaxies all lie within the CANDELS-EGS field, providing a dramatic illustration of the potential impact of field-to-field variance.
more3/2016
We present Hubble WFC3/IR slitless grism spectra of a remarkably bright z ≳ 10 galaxy candidate, GN-z11, identified initially from CANDELS/GOODS-N imaging data. A significant spectroscopic continuum break is detected at λ =1.47+/- 0.01 μ {{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 Lyα break redshifted to {z}{grism}={11.09}-0.12+0.08, just ˜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 3× larger than {L}* measured at z ˜ 6-8. The Spitzer IRAC detections up to 4.5 μm of this galaxy are consistent with a stellar mass of ˜109 M&sun;. This spectroscopic redshift measurement suggests that 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.
more12/2015
The IRAC ultradeep field and IRAC Legacy over GOODS programs are two ultradeep imaging surveys at 3.6 and 4.5 μm with the Spitzer Infrared Array Camera (IRAC). The primary aim is to directly detect the infrared light of reionization epoch galaxies at z > 7 and to constrain their stellar populations. The observations cover the Hubble Ultra Deep Field (HUDF), including the two HUDF parallel fields, and the CANDELS/GOODS-South, and are combined with archival data from all previous deep programs into one ultradeep data set. The resulting imaging reaches unprecedented coverage in IRAC 3.6 and 4.5 μm ranging from >50 hr over 150 arcmin2, >100 hr over 60 sq arcmin2, to ˜200 hr over 5-10 arcmin2. This paper presents the survey description, data reduction, and public release of reduced mosaics on the same astrometric system as the CANDELS/GOODS-South Wide Field Camera 3 (WFC3) data. To facilitate prior-based WFC3+IRAC photometry, we introduce a new method to create high signal-to-noise PSFs from the IRAC data and reconstruct the complex spatial variation due to survey geometry. The PSF maps are included in the release, as are registered maps of subsets of the data to enable reliability and variability studies. Simulations show that the noise in the ultradeep IRAC images decreases approximately as the square root of integration time over the range 20-200 hr, well below the classical confusion limit, reaching 1σ point-source sensitivities as faint as 15 nJy (28.5 AB) at 3.6 μm and 18 nJy (28.3 AB) at 4.5 μm. The value of such ultradeep IRAC data is illustrated by direct detections of z = 7-8 galaxies as faint as HAB = 28. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. Based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.
more10/2015
Thomson optical depth τ measurements from Planck provide new insights into the reionization of the universe. In pursuit of model-independent constraints on the properties of the ionizing sources, we determine the empirical evolution of the cosmic ionizing emissivity. We use a simple two-parameter model to map out the evolution in the emissivity at z ≳ 6 from the new Planck optical depth τ measurements, from the constraints provided by quasar absorption spectra and from the prevalence of Lyα emission in z ˜ 7-8 galaxies. We find the redshift evolution in the emissivity {\dot{N}}{ion}(z) required by the observations to be {({\text{}}d{{log}}10 {\dot{N}}{ion}/{dz})}z=8=-{0.15}-0.11+0.08 ({({\text{}}d{{log}}10 {\dot{N}}{ion}/{dz})}z=8=-{0.19}-0.11+0.09 for a flat prior), largely independent of the assumed clumping factor CH ii and entirely independent of the nature of the ionizing sources. The trend in {\dot{N}}{ion}(z) is well-matched by the evolution of the galaxy UV-luminosity density (d{{log}}10{ρ }{UV}/{dz}=-0.11+/- 0.04) to a magnitude limit ≳-13 mag, suggesting that galaxies are the sources that drive the reionization of the universe. The role of galaxies is further strengthened by the conversion from the UV luminosity density ρUV to {\dot{N}}{ion}(z) being possible for physically plausible values of the escape fraction fesc, the Lyman-continuum photon production efficiency ξion, and faint-end cut-off Mlim to the luminosity function. Quasars/active galactic nuclei appear to match neither the redshift evolution nor normalization of the ionizing emissivity. Based on the inferred evolution in the ionizing emissivity, we estimate that the z ˜ 10 UV-Iuminosity density is 8-4+15× lower than at z ˜ 6, consistent with the observations. The present approach of contrasting the inferred evolution of the ionizing emissivity with that of the galaxy UV luminosity density adds to the growing observational evidence that faint, star-forming galaxies drive the reionization of the universe. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.
more7/2015
We search the complete Hubble Frontier Field data set of Abell 2744 and its parallel field for z∼ 10 sources to further refine the evolution of the cosmic star formation rate density (SFRD) between z∼ 8 and z∼ 10. We independently confirm two images of the recently discovered triply imaged z∼ 9.8 source by Zitrin et al. and set an upper limit for similar z∼ 10 galaxies with red colors of {J}125-{H}160\gt 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 an SFRD which is consistent within the uncertainties with previous estimates at z∼ 10 from blank fields. The combination of these new results with all other estimates is also 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 ΛCDM. 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 data set to probe the evolution of the galaxy population at z\gt 8 before the advent of the James Webb Space Telescope. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.
more7/2015
Recently, a small sample of six z ˜ 9-10 candidates was discovered in CANDELS that are ˜ 10-20× more luminous than any of the previous z ˜ 9-10 galaxies identified over the HUDF/XDF and CLASH fields. We measure the sizes of these candidates to map out the size evolution of galaxies from the earliest observable times. Their sizes are also used to provide a valuable constraint on whether these unusual galaxy candidates are at high redshift. Using galfit to derive sizes from the CANDELS F160W images of these candidates, we find a mean size of 0\buildrel{\prime\prime}\over{.} 13+/- 0\buildrel{\prime\prime}\over{.} 02 (or 0.5 ± 0.1 kpc at z ˜ 9-10). This handsomely matches the 0.6 kpc size expected extrapolating lower-redshift measurements to z ˜ 9-10, while being much smaller than the 0.″59 mean size for lower-redshift interlopers to z ˜ 9-10 photometric selections lacking the blue IRAC color criterion. This suggests that source size may be an effective constraint on contaminants from z ˜ 9-10 selections lacking IRAC data. Assuming on the basis of the strong photometric evidence that the Oesch et al. sample is entirely at z ˜ 9-10, we can use this sample to extend current constraints on the size-luminosity, size-mass relation, and size evolution of galaxies to z˜ 10. We find that the z ˜ 9-10 candidate galaxies have broadly similar sizes and luminosities as z˜ 6-8 counterparts with star formation rate surface densities in the range of {{{Σ }}}{SFR}=1-20 {M}&sun; {{yr}}-1 {{kpc}}-2. The stellar mass-size relation is uncertain, but shallower than those inferred for lower-redshift galaxies. In combination with previous size measurements at z = 4-7, we find a size evolution of {(1+z)}-m with m=1.0+/- 0.1 for \gt 0.3L{*}z=3 galaxies, consistent with the evolution previously derived from 2\lt z\lt 8 galaxies.
more5/2015
We present a spectroscopic redshift measurement of a very bright Lyman break galaxy at z=7.7302+/- 0.0006 using the Keck/Multi-Object Spectrometer for Infra-Red Exploration. The source was pre-selected photometrically in the EGS field as a robust z ˜ 8 candidate with H = 25.0 mag based on optical non-detections and a very red Spitzer/IRAC [3.6]-[4.5] broad-band color driven by high equivalent width [O iii]+Hβ line emission. The Lyα line is reliably detected at 6.1σ and shows an asymmetric profile as expected for a galaxy embedded in a relatively neutral intergalactic medium near the Planck peak of cosmic reionization. The line has a rest-frame equivalent width of E{{W}0}=21+/- 4 Å and is extended with {{V}FWHM}=360-70+90 km s-1. The source is perhaps the brightest and most massive z ˜ 8 Lyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having already assembled {{10}9.9+/- 0.2} {{M}&sun; } of stars at only 650 Myr after the Big Bang. The spectroscopic redshift measurement sets a new redshift record for galaxies. This enables reliable constraints on the stellar mass, star formation rate, and formation epoch, as well as combined [O iii]+Hβ line equivalent widths. The redshift confirms that the IRAC [4.5] photometry is very likely dominated by line emission with EW0([O iii]+Hβ) = 720-150+180 Å. This detection thus adds to the evidence that extreme rest-frame optical emission lines are a ubiquitous feature of early galaxies promising very efficient spectroscopic follow-up in the future with infrared spectroscopy using the James Webb Space Telescope and, later, ELTs.
more4/2015
The remarkable Hubble Space Telescope (HST) data sets from the CANDELS, HUDF09, HUDF12, ERS, and BoRG/HIPPIES programs have allowed us to map the evolution of the rest-frame UV luminosity function (LF) from z˜ 10 to z˜ 4. We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5859, 3001, 857, 481, 217, and 6 galaxy candidates at z˜ 4, z˜ 5, z˜ 6, z˜ 7, z˜ 8, and z˜ 10, respectively, from the ˜1000 arcmin2 area covered by these data sets. This sample of >10,000 galaxy candidates at z≥slant 4 is by far the largest assembled to date with HST. The selection of z ˜ 4-8 candidates over the five CANDELS fields allows us to assess the cosmic variance; the largest variations are at z≥slant 7. Our new LF determinations at z˜ 4 and z˜ 5 span a 6 mag baseline and reach to -16 AB mag. These determinations agree well with previous estimates, but the larger samples and volumes probed here result in a more reliable sampling of \gt {{L}*} galaxies and allow us to reassess the form of the UV LFs. Our new LF results strengthen our earlier findings to 3.4σ significance for a steeper faint-end slope of the UV LF at z\gt 4, with α evolving from α =-1.64+/- 0.04 at z˜ 4 to α =-2.06+/- 0.13 at z˜ 7 (and α =-2.02+/- 0.23 at z˜ 8), consistent with that expected from the evolution of the halo mass function. We find less evolution in the characteristic magnitude M* from z˜ 7 to z˜ 4; the observed evolution in the LF is now largely represented by changes in {{&phis; }*}. No evidence for a non-Schechter-like form to the z ˜ 4-8 LFs is found. A simple conditional LF model based on halo growth and evolution in the M/L ratio (\propto {{(1+z)}-1.5}) of halos provides a good representation of the observed evolution. Based on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/IRFU, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. This work is based in part on data products produced at Terapix available at the Canadian Astronomy Data Centre as part of the Canada-France-Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.
more10/2014
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.
more5/2014
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.
more11/2013
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. .
more11/2013
Using new ultradeep Spitzer/InfraRed Array Camera (IRAC) photometry from the IRAC Ultra Deep Field program, we investigate the stellar populations of a sample of 63 Y-dropout galaxy candidates at z ~ 8, only 650 Myr after the big bang. The sources are selected from HST/ACS+WFC3/IR data over the Hubble Ultra Deep Field (HUDF), two HUDF parallel fields, and wide area data over the CANDELS/GOODS-South. The new Spitzer/IRAC data increase the coverage in [3.6] and [4.5] to ~120h over the HUDF reaching depths of ~28 (AB,1σ). The improved depth and inclusion of brighter candidates result in direct >=3σ InfraRed Array Camera (IRAC) detections of 20/63 sources, of which 11/63 are detected at >=5σ. The average [3.6]-[4.5] colors of IRAC detected galaxies at z ~ 8 are markedly redder than those at z ~ 7, observed only 130 Myr later. The simplest explanation is that we witness strong rest-frame optical emission lines (in particular [O III] λλ4959, 5007 + Hβ) moving through the IRAC bandpasses with redshift. Assuming that the average rest-frame spectrum is the same at both z ~ 7 and z ~ 8 we estimate a rest-frame equivalent width of {W}_{[O\,\scriptsize{III}]\ \lambda \lambda 4959,5007+H\beta }=670^{+260}_{-170} Å contributing 0.56^{+0.16}_{-0.11} mag to the [4.5] filter at z ~ 8. The corresponding {W}_{H\alpha }=430^{+160}_{-110} Å implies an average specific star formation rate of sSFR=11_{-5}^{+11} Gyr-1 and a stellar population age of 100_{-50}^{+100} Myr. Correcting the spectral energy distribution for the contribution of emission lines lowers the average best-fit stellar masses and mass-to-light ratios by ~3 ×, decreasing the integrated stellar mass density to \rho ^*(z=8,M_{\rm{UV}}<-18)=0.6^{+0.4}_{-0.3}\times 10^6 \,M_\odot Mpc-3. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #11563, 9797. Based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through contract 125790 issued by JPL/Caltech. Based on service mode observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 073.A-0764A). Based on data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.
more8/2013
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.
more8/2013
We present a study of rest-frame UV-to-optical color distributions for z ~ 4 galaxies based on the combination of deep HST/ACS+WFC3/IR data with Spitzer/IRAC imaging. In particular, we use new, ultra-deep data from the IRAC Ultradeep Field program (IUDF10), together with previous, public IRAC data over the GOODS fields. Our sample contains a total of ~2600 galaxies selected as B-dropout Lyman-break Galaxies in the HUDF and its deep parallel field HUDF09-2, as well as GOODS-North/South. This sample is used to investigate the UV continuum slopes β and Balmer break colors (J 125 - [4.5]) as a function of rest-frame optical luminosity (using [4.5] to avoid optical emission lines). We find that galaxies at Mz < -21.5 (roughly corresponding to L^*_{z\sim 4}) are significantly redder than their lower luminosity counterparts. The UV continuum slopes and the J 125 - [4.5] colors are well correlated, indicating that the dust reddening at these redshifts is better described by an SMC-like extinction curve, rather than the typically assumed Calzetti reddening. After dust correction, we find that the galaxy population shows mean stellar population ages in the range 108.5 to 109 yr, with a dispersion of ~0.5 dex, and only weak trends as a function of luminosity. Only a small fraction of galaxies shows Balmer break colors consistent with extremely young ages, younger than 100 Myr. Under the assumption of smooth star-formation histories, this fraction is 12%-19% for galaxies at Mz < -19.75. Our results are consistent with a gradual build-up of stars and dust in galaxies at z > 4 with only a small fraction of stars being formed in short, intense bursts of star-formation. 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.
more3/2013
Ultra-deep WFC3/IR observations on the HUDF from the HUDF09 program revealed just one plausible z ~ 10 candidate, UDFj-39546284. UDFj-39546284 had all the properties expected of a galaxy at z ~ 10 showing (1) no detection in the deep ACS+WFC3 imaging data blueward of the F160W band, exhibiting (2) a blue spectral slope redward of the break, and showing (3) no prominent detection in deep IRAC observations. The new, similarly deep WFC3/IR HUDF12 F160W observations over the HUDF09/XDF allow us to further assess this candidate. These observations show that this candidate, previously only detected at ~5.9σ in a single band, clearly corresponds to a real source. It is detected at ~5.3σ in the new H 160-band data and at ~7.8σ in the full 85-orbit H 160-band stack. Interestingly, the non-detection of the source (<1σ) in the new F140W observations suggests a higher redshift. Formally, the best-fit redshift of the source utilizing all the WFC3+ACS (and IRAC+Ks -band) observations is 11.8 ± 0.3. However, we consider the z ~ 12 interpretation somewhat unlikely, since the source would either need to be ~20× more luminous than expected or show very high-EW Lyα emission (which seems improbable given the extensive neutral gas prevalent early in the reionization epoch). Lower-redshift solutions fail if only continuum models are allowed. Plausible lower-redshift solutions require that the H 160-band flux be dominated by line emission such as Hα or [O III] with extreme EWs. The tentative detection of line emission at 1.6 μm in UDFj-39546284 in a companion paper suggests that such emission may have already been found. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained by the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
more3/2013
We present deep WFC3 grism observations of the candidate z ~ 12 galaxy UDFj-39546284 in the Hubble Space Telescope (HST) Ultra Deep Field (UDF), by combining spectroscopic data from the 3D-HST and CANDELS surveys. The total exposure time is 40.5 ks and the spectrum covers 1.10 < λ < 1.65 μm. We search for faint emission lines by cross-correlating the two-dimensional G141 spectrum with the observed H 160 morphology, a technique that is unique to slitless spectroscopy at HST resolution. We find a 2.7σ detection of an emission line at 1.599 μm—just redward of the JH 140 filter—with flux 3.5 ± 1.3 × 10-18 erg s-1 cm-2. Assuming that the line is real, it contributes 110% ± 40% of the observed H 160 flux and has an observed equivalent width >7300 Å. If the line is confirmed, it could be Lyα at z = 12.12. However, a more plausible interpretation, given current results, could be a lower redshift feature such as [O III]λ4959,5007 at z = 2.19. We find two other 3D-HST [O III] emitters within 1000 km s-1 of that redshift in the GOODS-South field. Additional support for this interpretation comes from the discovery of a bright "[O III] blob" with a secure G141 grism redshift of z = 1.605. This object has a strikingly large observed equivalent width of nearly 9000 Å that results in similar "dropout" colors as UDFj-39546284. Based on observations made with the NASA/ESA Hubble Space Telescope, programs GO-12099, 12177, and 12547, obtained 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.
more11/2012
We present new z ~ 8 galaxy candidates from a search over ~95 arcmin2 of WFC3/IR data, tripling the previous search area for bright z ~ 8 galaxies. Our analysis uses newly acquired WFC3/IR imaging data from the CANDELS Multi-Cycle Treasury program over the GOODS-South field. These new data are combined with existing deep optical Advanced Camera for Surveys (ACS) imaging to search for relatively bright (M UV < -19.5 mag) z ~ 8 galaxy candidates using the Lyman break technique. These new candidates are used to determine the bright end of the UV luminosity function (LF) of star-forming galaxies at z ~ 7.2-8.7, i.e., a cosmic age of 600 ± 80 Myr. To minimize contamination from lower redshift galaxies, we make full use of all optical ACS data and impose strict non-detection criteria based on an optical χ2 opt flux measurement. In the whole search area, we identify 16 candidate z ~ 8 galaxies, spanning a magnitude range H 160, AB = 25.7-27.9 mag. The new data show that the UV LF is a factor ~1.7 lower at M UV < -19.5 mag than determined from the HUDF09 and Early Release Science (ERS) data alone. Combining this new sample with the previous candidates from the HUDF09 and ERS data allows us to perform the most accurate measurement of the z ~ 8 UV LF yet. Schechter function fits to the combined data result in a best-fit characteristic magnitude of M *(z = 8) = -20.04 ± 0.46 mag. The faint-end slope is very steep, though quite uncertain, with α = -2.06 ± 0.32. A combination of wide-area data with additional ultra-deep imaging will be required to significantly reduce the uncertainties on these parameters in the future. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. for NASA under contract NAS5-26555.
more9/2012
We combine recent estimates of dust extinction at z ~ 4-7 with UV luminosity function (LF) determinations to derive star formation rate (SFR) functions at z ~ 4-7. SFR functions provide a more physical description of galaxy buildup at high redshift and allow for direct comparisons to SFRs at lower redshifts determined by a variety of techniques. Our SFR functions are derived from well-established z ~ 4-7 UV LFs, UV-continuum slope trends with redshift and luminosity, and infrared excess (IRX)-β relations. They are well described by Schechter relations. We extend the comparison baseline for SFR functions to z ~ 2 by considering recent determinations of the Hα and mid-IR LFs. The low-end slopes of the SFR functions are flatter than for the UV LFs, Δα ~ +0.13, and show no clear evolution with cosmic time (z ~ 0-7). In addition, we find that the characteristic value SFR* from the Schechter fit to the SFR function exhibits consistent, and substantial, linear growth as a function of redshift from ~5 M &sun; yr-1 at z ~ 8, 650 Myr after the big bang, to ~100 M &sun; yr-1 at z ~ 2, ~2.5 Gyr later. Recent results at z ~ 10, close to the onset of galaxy formation, are consistent with this trend. The uniformity of this evolution is even greater than seen in the UV LF over the redshift range z ~ 2-8, providing validation for our dust corrections. These results provide strong evidence that galaxies build up uniformly over the first 3 Gyr of cosmic time.
more6/2012
The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LFs) from these new HUDF09 data for 132 z ~ 7 and z ~ 8 galaxies are combined with new LFs for z ~ 5-6 galaxies and the earlier z ~ 4 LF to reach to very faint limits (<0.05 L* z = 3). The faint-end slopes α are steep: -1.79 ± 0.12 (z ~ 5), -1.73 ± 0.20 (z ~ 6), -2.01 ± 0.21 (z ~ 7), and -1.91 ± 0.32 (z ~ 8). Slopes α <~ -2 lead to formally divergent UV fluxes, though galaxies are not expected to form below ~ - 10 AB mag. These results have important implications for reionization. The weighted mean slope at z ~ 6-8 is -1.87 ± 0.13. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. While current results show that galaxies can reionize the universe by z ~ 6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z > 8, taking α to be -1.87 ± 0.13 (the mean value at z ~ 6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061+0.009 - 0.006. However, this result will change if the faint-end slope α is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (dα/dz ~ -0.05 ± 0.04) that would increase the Thomson optical depths to 0.079+0.063 - 0.017, consistent with recent WMAP estimates (τ = 0.088 ± 0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the UV ionizing flux from faint galaxies. 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.
more3/2012
We report the discovery of seven strongly lensed Lyman-break galaxy (LBG) candidates at z ~ 7 detected in Hubble Space Telescope Wide Field Camera 3 (WFC3) imaging of A1703. The brightest candidate, called A1703-zD1, has an observed (lensed) magnitude of 24.0 AB (26σ) in the WFC3/IR F160W band, making it 0.2 mag brighter than the z 850-dropout candidate recently reported behind the Bullet Cluster and 0.7 mag brighter than the previously brightest known z ~ 7.6 galaxy, A1689-zD1. With a cluster magnification of ~9, this source has an intrinsic magnitude of H 160 = 26.4 AB, a strong z 850 - J 125 break of 1.7 mag, and a photometric redshift of z ~ 6.7. Additionally, we find six other bright LBG candidates with H 160-band magnitudes of 24.9-26.4, photometric redshifts z ~ 6.4 - 8.8, and magnifications μ ~ 3-40. Stellar population fits to the Advanced Camera for Surveys, WFC3/IR, and Spitzer/Infrared Array Camera data for A1703-zD1 and A1703-zD4 yield stellar masses (0.7 - 3.0) × 109 M &sun;, stellar ages 5-180 Myr, and star formation rates ~7.8 M &sun; yr-1, and low reddening with AV <= 0.7. The source-plane reconstruction of the exceptionally bright candidate A1703-zD1 exhibits an extended structure, spanning ~4 kpc in the z ~ 6.7 source plane, and shows three resolved star-forming knots of radius r ~ 0.4 kpc. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy under NASA contract NAS5-26555. Based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407.
more2/2012
We search for z ~ 10 galaxies over ~160 arcmin2 of Wide-Field Camera 3 (WFC3)/IR data in the Chandra Deep Field South, using the public HUDF09, Early Release Science, and CANDELS surveys, that reach to 5σ depths ranging from 26.9 to 29.4 in H 160 AB mag. z >~ 9.5 galaxy candidates are identified via J 125 - H 160 > 1.2 colors and non-detections in any band blueward of J 125. Spitzer Infrared Array Camera (IRAC) photometry is key for separating the genuine high-z candidates from intermediate-redshift (z ~ 2-4) galaxies with evolved or heavily dust obscured stellar populations. After removing 16 sources of intermediate brightness (H 160 ~ 24-26 mag) with strong IRAC detections, we only find one plausible z ~ 10 galaxy candidate in the whole data set, previously reported in Bouwens et al.. The newer data cover a 3 × larger area and provide much stronger constraints on the evolution of the UV luminosity function (LF). If the evolution of the z ~ 4-8 LFs is extrapolated to z ~ 10, six z ~ 10 galaxies are expected in our data. The detection of only one source suggests that the UV LF evolves at an accelerated rate before z ~ 8. The luminosity density is found to increase by more than an order of magnitude in only 170 Myr from z ~ 10 to z ~ 8. This increase is >=4 × larger than expected from the lower redshift extrapolation of the UV LF. We are thus likely witnessing the first rapid buildup of galaxies in the heart of cosmic reionization. Future deep Hubble Space Telescope WFC3/IR data, reaching to well beyond 29 mag, can enable a more robust quantification of the accelerated evolution around z ~ 10. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc., for NASA under contract NAS5-26555. Partially based on observations made with the Spitzer Space Telescope, operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.
more8/2011
We identify 73 z ~ 7 and 59 z ~ 8 candidate galaxies in the reionization epoch, and use this large 26-29.4 AB mag sample of galaxies to derive very deep luminosity functions to < - 18 AB mag and the star formation rate (SFR) density at z ~ 7 and z ~ 8 (just 800 Myr and 650 Myr after recombination, respectively). The galaxy sample is derived using a sophisticated Lyman-break technique on the full two-year Wide Field Camera 3/infrared (WFC3/IR) and Advanced Camera for Surveys (ACS) data available over the HUDF09 (~29.4 AB mag, 5σ), two nearby HUDF09 fields (~29 AB mag, 5σ, 14 arcmin2), and the wider area Early Release Science (~27.5 AB mag, 5σ, ~40 arcmin2). The application of strict optical non-detection criteria ensures the contamination fraction is kept low (just ~7% in the HUDF). This very low value includes a full assessment of the contamination from lower redshift sources, photometric scatter, active galactic nuclei, spurious sources, low-mass stars, and transients (e.g., supernovae). From careful modeling of the selection volumes for each of our search fields, we derive luminosity functions for galaxies at z ~ 7 and z ~ 8 to < - 18 AB mag. The faint-end slopes α at z ~ 7 and z ~ 8 are uncertain but very steep at α = -2.01 ± 0.21 and α = -1.91 ± 0.32, respectively. Such steep slopes contrast to the local α >~ -1.4 and may even be steeper than that at z ~ 4 where α = -1.73 ± 0.05. With such steep slopes (α <~ -1.7) lower luminosity galaxies dominate the galaxy luminosity density during the epoch of reionization. The SFR densities derived from these new z ~ 7 and z ~ 8 luminosity functions are consistent with the trends found at later times (lower redshifts). We find reasonable consistency with the SFR densities implied from reported stellar mass densities being only ~40% higher at z < 7. This suggests that (1) the stellar mass densities inferred from the Spitzer Infrared Array Camera (IRAC) photometry are reasonably accurate and (2) that the initial mass function at very high redshift may not be very different from that at later times. Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs 11563, 9797, and 10632.
more1/2011
Searches for very-high-redshift galaxies over the past decade have yielded a large sample of more than 6,000 galaxies existing just 900-2,000million years (Myr) after the Big Bang (redshifts 6>z>3 ref. 1). The Hubble Ultra Deep Field (HUDF09) data have yielded the first reliable detections of z~8 galaxies that, together with reports of a γ-ray burst at z~8.2 (refs 10, 11), constitute the earliest objects reliably reported to date. Observations of z~7-8 galaxies suggest substantial star formation at z>9-10 (refs 12, 13). Here we use the full two-year HUDF09 data to conduct an ultra-deep search for z~10 galaxies in the heart of the reionization epoch, only 500Myr after the Big Bang. Not only do we find one possible z~10 galaxy candidate, but we show that, regardless of source detections, the star formation rate density is much smaller (~10%) at this time than it is just ~200Myr later at z~8. This demonstrates how rapid galaxy build-up was at z~10, as galaxies increased in both luminosity density and volume density from z~10 to z~8. The 100-200Myr before z~10 is clearly a crucial phase in the assembly of the earliest galaxies.
more