R. A. A. Bowler

NEW CONSTRAINTS ON COSMIC REIONIZATION FROM THE 2012 HUBBLE ULTRA DEEP FIELD CAMPAIGN

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign has acquired the deepest infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicate reionization was underway. The UDF12 campaign thus provides the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize the new UDF12 results with the most recent constraints from CMB observations to infer redshift-dependent ultraviolet (UV) luminosity densities, reionization histories, and electron scattering optical depth evolution consistent with the available data. Under reasonable assumptions about the escape fraction of hydrogen-ionizing photons and the intergalactic medium clumping factor, we find that to fully reionize the universe by redshift z ~ 6 the population of star-forming galaxies at redshifts z ~ 7-9 likely must extend in luminosity below the UDF12 limits to absolute UV magnitudes of M UV ~ –13 or fainter. Moreover, low levels of star formation extending to redshifts z ~ 15-25, as suggested by the normal UV colors of z ≃ 7-8 galaxies and the smooth decline in abundance with redshift observed by UDF12 to z ≃ 10, are additionally likely required to reproduce the optical depth to electron scattering inferred from CMB observations.

A new multifield determination of the galaxy luminosity function at z = 7-9 incorporating the 2012 Hubble Ultra-Deep Field imaging

We present a new determination of the ultraviolet (UV) galaxy luminosity function (LF) at redshift z ≃ 7 and 8, and a first estimate at z ≃ 9. An accurate determination of the form and evolution of the galaxy LF during this era is of key importance for improving our knowledge of the earliest phases of galaxy evolution and the process of cosmic reionization. Our analysis exploits to the full the new, deepest Wide Field Camera 3/infrared imaging from our Hubble Space Telescope (HST) Ultra-Deep Field 2012 (UDF12) campaign, with dynamic range provided by including a new and consistent analysis of all appropriate, shallower/wider area HST survey data. Our new measurement of the evolving LF at z ≃ 7 to 8 is based on a final catalogue of ≃600 galaxies, and involves a step-wise maximum-likelihood determination based on the photometric redshift probability distribution for each object; this approach makes full use of the 11-band imaging now available in the Hubble Ultra-Deep Field (HUDF), including the new UDF12 F140W data, and the latest Spitzer IRAC imaging. The final result is a determination of the z ≃ 7 LF extending down to UV absolute magnitudes M_1500 = −16.75 (AB mag) and the z ≃ 8 LF down to M_1500 = −17.00. Fitting a Schechter function, we find M*_1500 = −19.90^(+0.23)_(−0.28), log ϕ* = −2.96^(+0.18)_(−0.23) and a faint-end slope α = −1.90^(+0.14)_(−0.15) at z ≃ 7, and M*_1500 = −20.12^(+0.37)_(−0.48), log ϕ* = −3.35^(+0.28)_(−0.47) and α = −2.02^(+0.22)_(-0.23) at z ≃ 8. These results strengthen previous suggestions that the evolution at z > 7 appears more akin to ‘density evolution’ than the apparent ‘luminosity evolution’ seen at z ≃ 5 − 7. We also provide the first meaningful information on the LF at z ≃ 9, explore alternative extrapolations to higher redshifts, and consider the implications for the early evolution of UV luminosity density. Finally, we provide catalogues (including derived z_phot, M_1500 and photometry) for the most robust z ∼ 6.5-11.9 galaxies used in this analysis. We briefly discuss our results in the context of earlier work and the results derived from an independent analysis of the UDF12 data based on colour–colour selection.

THE UV LUMINOSITY FUNCTION OF STAR-FORMING GALAXIES VIA DROPOUT SELECTION AT REDSHIFTS z ∼ 7 AND 8 FROM THE 2012 ULTRA DEEP FIELD CAMPAIGN

We present a catalog of high-redshift star-forming galaxies selected to lie within the redshift range z ≃ 7-8 using the Ultra Deep Field 2012 (UDF12), the deepest near-infrared (near-IR) exposures yet taken with the Hubble Space Telescope (HST). As a result of the increased near-IR exposure time compared to previous HST imaging in this field, we probe ~0.65 (0.25) mag fainter in absolute UV magnitude, at z ~ 7 (8), which increases confidence in a measurement of the faint end slope of the galaxy luminosity function. Through a 0.7 mag deeper limit in the key F105W filter that encompasses or lies just longward of the Lyman break, we also achieve a much-refined color-color selection that balances high redshift completeness and a low expected contamination fraction. We improve the number of dropout-selected UDF sources to 47 at z ~ 7 and 27 at z ~ 8. Incorporating brighter archival and ground-based samples, we measure the z ≃ 7 UV luminosity function to an absolute magnitude limit of M_(UV) = –17 and find a faint end Schechter slope of ɑ =-1.87^(+0.18)_(-0.17). Using a similar color-color selection at z ≃ 8 that takes our newly added imaging in the F140W filter into account, and incorporating archival data from the HIPPIES and BoRG campaigns, we provide a robust estimate of the faint end slope at z ≃ 8, ɑ =-1.94^(+0.21)_(-0.24). We briefly discuss our results in the context of earlier work and that derived using the same UDF12 data but with an independent photometric redshift technique.

The UV continua and inferred stellar populations of galaxies at z ≃ 7–9 revealed by the Hubble Ultra-Deep Field 2012 campaign

We use the new ultra-deep, near-infrared imaging of the Hubble Ultra-Deep Field (HUDF) provided by our UDF12 Hubble Space Telescope (HST) Wide Field Camera 3/IR campaign to explore the rest-frame ultraviolet (UV) properties of galaxies at redshifts z > 6.5. We present the first unbiased measurement of the average UV power-law index, 〈β〉, (fλ ∝ λ^β) for faint galaxies at z ≃ 7, the first meaningful measurements of 〈β〉 at z ≃ 8, and tentative estimates for a new sample of galaxies at z ≃ 9. Utilizing galaxy selection in the new F140W (J_140) imaging to minimize colour bias, and applying both colour and power-law estimators of β, we find 〈β〉 = −2.1 ± 0.2 at z ≃ 7 for galaxies with M_UV ≃ −18. This means that the faintest galaxies uncovered at this epoch have, on average, UV colours no more extreme than those displayed by the bluest star-forming galaxies at low redshift. At z ≃ 8 we find a similar value, 〈β〉 = −1.9 ± 0.3. At z ≃ 9, we find 〈β〉 = −1.8 ± 0.6, essentially unchanged from z ≃ 6 to 7 (albeit highly uncertain). Finally, we show that there is as yet no evidence for a significant intrinsic scatter in β within our new, robust z ≃ 7 galaxy sample. Our results are most easily explained by a population of steadily star-forming galaxies with either ≃ solar metallicity and zero dust, or moderately sub-solar (≃10–20 per cent) metallicity with modest dust obscuration (AV ≃ 0.1–0.2). This latter interpretation is consistent with the predictions of a state-of-the-art galaxy-formation simulation, which also suggests that a significant population of very-low metallicity, dust-free galaxies with β ≃ −2.5 may not emerge until M_UV > −16, a regime likely to remain inaccessible until the James Webb Space Telescope.

A robust sample of galaxies at redshifts 6.0<z<8.7: Stellar populations, star formation rates and stellar masses

We present the results of a photometric redshift analysis designed to identify z≥ 6 galaxies from the near-infrared Hubble Space Telescope imaging in three deep fields [Hubble Ultra Deep Field (HUDF), HUDF09-2 and Early Release Science] covering a total area of 45 square arcmin. By adopting a rigorous set of criteria for rejecting low-redshift interlopers, and by employing a deconfusion technique to allow the available ultradeep IRAC imaging to be included in the candidate-selection process, we have derived a robust sample of 70 Lyman break galaxies (LBGs) spanning the redshift range 6.0 < z < 8.7. Based on our final sample, we investigate the distribution of ultraviolet (UV) spectral slopes (f_λ∝λ^β), finding a variance-weighted mean value of 〈β〉=−2.05 ± 0.09 which, contrary to some previous results, is not significantly bluer than displayed by lower redshift starburst galaxies. We confirm the correlation between UV luminosity and stellar mass reported elsewhere, but based on fitting galaxy templates featuring a range of star formation histories (SFHs), metallicities and reddening, we find that, at z≥ 6, the range in mass-to-light ratio (M_★/L_(UV)) at a given UV luminosity could span a factor of ≃50. Focusing on a subsample of 21 candidates with IRAC detections at 3.6µm, we find that L^★ LBGs at z≃ 6.5 have a median stellar mass of M_★= (2.1 ± 1.1) × 10^9 M_⊙ (Chabrier initial mass function) and a median specific star formation rate (sSFR) of 1.9 ± 0.8 Gyr^(−1). Using the same subsample, we have investigated the influence of nebular continuum and line emission, finding that for the majority of candidates (16 out of 21), the best-fitting stellar masses are reduced by less than a factor of 2.5. However, galaxy template fits exploring a plausible range of SFHs and metallicities provide no compelling evidence of a clear connection between SFR and stellar mass at these redshifts. Finally, a detailed comparison of our final sample with the results of previous studies suggests that, at faint magnitudes, several high-redshift galaxy samples in the literature are significantly contaminated by low-redshift interlopers.

The bright end of the galaxy luminosity function at z =7: before the onset of mass quenching?

We present the results of a new search for bright star-forming galaxies at redshift z ≃ 7 within the UltraVISTA second data release (DR2) and UKIDSS (UKIRT Infrared Deep Sky Survey) UDS (Ultra Deep Survey) DR10 data, which together provide 1.65 deg^2 of near-infrared imaging with overlapping optical and Spitzer data. Using a full photometric redshift analysis, to identify high-redshift galaxies and reject contaminants, we have selected a sample of 34 luminous (−22.7<M_(UV)<−21.2 −22.7<M_(UV)<−21.2) galaxies with 6.5 < z < 7.5. Crucially, the deeper imaging provided by UltraVISTA DR2 confirms all of the robust objects previously uncovered by Bowler et al., validating our selection technique. Our new expanded galaxy sample includes the most massive galaxies known at z ≃ 7, with M^* ≃ 10^(10) M_⊙, and the majority are resolved, consistent with larger sizes (r1/2 ≃ 1–1.5 kpc) than displayed by less massive galaxies. From our final robust sample, we determine the form of the bright end of the rest-frame UV galaxy luminosity function (LF) at z ≃ 7, providing strong evidence that it does not decline as steeply as predicted by the Schechter-function fit to fainter data. We exclude the possibility that this is due to either gravitational lensing, or significant contamination of our galaxy sample by active galactic nuclei (AGN). Rather, our results favour a double power-law form for the galaxy LF at high redshift, or, more interestingly, an LF which simply follows the form of the dark matter halo mass function at bright magnitudes. This suggests that the physical mechanism which inhibits star formation activity in massive galaxies (i.e. AGN feedback or some other form of ‘mass quenching’) has yet to impact on the observable galaxy LF at z ≃ 7, a conclusion supported by the estimated masses of our brightest galaxies which have only just reached a mass comparable to the critical ‘quenching mass’ of M^* ≃ 10^(10).2 M_⊙ derived from studies of the mass function of star-forming galaxies at lower redshift.

EVOLUTION OF THE SIZES OF GALAXIES OVER 7 <z< 12 REVEALED BY THE 2012 HUBBLE ULTRA DEEP FIELD CAMPAIGN

We analyze the redshift- and luminosity-dependent sizes of dropout galaxy candidates in the redshift range z ~ 7-12 using deep images from the 2012 Hubble Ultra Deep Field (UDF12) campaign, which offers two advantages over that used in earlier work. First, we utilize the increased signal-to-noise ratio offered by the UDF12 imaging to provide improved measurements for known galaxies at z ≃ 6.5-8 in the HUDF. Second, because the UDF12 data have allowed the construction of the first robust galaxy sample in the HUDF at z > 8, we have been able to extend the measurement of average galaxy size out to higher redshifts. Restricting our measurements to sources detected at >15σ, we confirm earlier indications that the average half-light radii of z ~ 7-12 galaxies are extremely small, 0.3-0.4 kpc, comparable to the sizes of giant molecular associations in local star-forming galaxies. We also confirm that there is a clear trend of decreasing half-light radius with increasing redshift, and provide the first evidence that this trend continues beyond z ≃ 8. Modeling the evolution of the average half-light radius as a power law, ∝(1 + z)^s , we obtain a best-fit index of s = -1.30^(+0.12)_(-0.14) over z ~ 4-12. A clear size-luminosity relation is evident in our dropout samples. This relation can be interpreted in terms of a constant surface density of star formation over a range in luminosity of 0.05-1.0L^*_(z=3). The average star formation surface density in dropout galaxies is 2-3 orders of magnitude lower than that found in extreme starburst galaxies, but is comparable to that seen today in the centers of normal disk galaxies.

THE 2012 HUBBLE ULTRA DEEP FIELD (UDF12): OBSERVATIONAL OVERVIEW

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

The galaxy luminosity function at z ≃ 6 and evidence for rapid evolution in the bright end from z ≃ 7 to 5

We present the results of a search for bright (-22.7 5. We calculate the bright end of the rest-frame Ultra-Violet (UV) luminosity function (LF) at z ~ 6. The galaxy number counts are a factor of ~1.7 lower than predicted by the recent LF determination by Bouwens et al.. In comparison to other smaller area studies, we find an evolution in the characteristic magnitude between z ~ 5 and z ~ 7 of dM* ~ 0.4 mag, and show that a double power-law or a Schechter function can equally well describe the LF at z = 6. Furthermore, the bright-end of the LF appears to steepen from z ~ 7 to z ~ 5, which could indicate the onset of mass quenching or the rise of dust obscuration, a conclusion supported by comparing the observed LFs to a range of theoretical model predictions.

Discovery of bright z ≃ 7 galaxies in the UltraVISTA survey

We have exploited the new, deep, near-infrared UltraVISTA imaging of the Cosmological Evolution Survey (COSMOS) field, in tandem with deep optical and mid-infrared imaging, to conduct a new search for luminous galaxies at redshifts z ≃ 7. The year-one UltraVISTA data provide contiguous Y, J, H, Ks imaging over 1.5 deg2, reaching a 5σ detection limit of Y + J ≃ 25 (AB mag, 2-arcsec-diameter aperture). The central ≃1 deg2 of this imaging coincides with the final deep optical (u*, g, r, i) data provided by the Canada–France–Hawaii Telescope (CFHT) Legacy Survey and new deep Subaru/Suprime-Cam z′-band imaging obtained specifically to enable full exploitation of UltraVISTA. It also lies within the Hubble Space Telescope (HST) I814 band and Spitzer/Infrared Array Camera imaging obtained as part of the COSMOS survey. We have utilized this unique multiwavelength dataset to select galaxy candidates at redshifts z > 6.5 by searching first for Y + J-detected objects which are undetected in the CFHT and HST optical data. This sample was then refined using a photometric redshift fitting code, enabling the rejection of lower redshift galaxy contaminants and cool galactic M, L, T dwarf stars. The final result of this process is a small sample of (at most) 10 credible galaxy candidates at z > 6.5 (from over 200 000 galaxies detected in the year-one UltraVISTA data) which we present in this paper. The first four of these appear to be robust galaxies at z > 6.5, and fitting to their stacked spectral energy distribution yields zphot = 6.98 ± 0.05 with a stellar mass M* ≃ 5 × 109 M⊙ and rest-frame ultraviolet (UV) spectral slope β ≃ −2.0 ± 0.2 (where fλ ∝ λβ). The next three are also good candidates for z > 6.5 galaxies, but the possibility that they are dwarf stars cannot be completely excluded. Our final subset of three additional candidates is afflicted not only by potential dwarf star contamination, but also contains objects likely to lie at redshifts just below z = 6.5. We show that the three even-brighter z ≳ 7 galaxy candidates reported in the COSMOS field by Capak et al. are in fact all lower redshift galaxies at z ≃ 1.5–3.5. Consequently the new z ≃ 7 galaxies reported here are the first credible z ≃ 7 Lyman-break galaxies discovered in the COSMOS field and, as the most UV luminous discovered to date at these redshifts, are prime targets for deep follow-up spectroscopy. We explore their physical properties, and briefly consider the implications of their inferred number density for the form of the galaxy luminosity function at z ≃ 7.