Abstract. I review some recent progress made in our understanding of galaxy evolution and the cosmic history of star formation. Like bookends, the results obtained from deep ground-based spectroscopy and from the Hubble Deep Field imaging survey put brackets around the intermediate redshift interval, 1 < z < 2, where starbirth probably peaked at a rate 10 times higher than today. The steady decline observed since z ∼ 1 is largely associated with late-type galaxies. At z > ∼ 2.5, the Lyman-break selected objects may represent the precursors of present-day spheroids, but appear, on average, quite underluminous relative to the expectations of the standard early-and-rapidly forming picture for spheroidal systems. The observed ultraviolet light density accounts for the bulk of the metals seen today in "normal" massive galaxies.

The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg 2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 z 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey -South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field 2009 and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z 3.5, including 113 at z ≃ 7-8. We fit the observed spectral energy distribution (SED) to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (β) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on β than other methods, as well as a reduced number of galaxies with catastrophically incorrect β measurements (i.e., ∆β > 1). We find that the median value of β evolves significantly from −1.82 +0.00 −0.04 at z = 4, to −2.37 +0.26 −0.06 at z = 7. Additionally, we find that faint galaxies at z = 7 have β = −2.68 +0.39 −0.24 (∼ −2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require "exotic" stellar populations (e.g., very-low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z = 7, to A V ∼ 0.5 mag at z = 4. The timescale for this increase is consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2σ) correlation between β and M UV when measuring M UV at a consistent restframe wavelength of 1500 Å. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between β and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z ≥ 7. Thus the stellar-massmetallicity relation, previously observed up to z ∼ 3, may extend out to z = 7 -8.

We report on the initial results of a spectroscopic investigation of galaxies in the Hubble Deep Field which exhibit spectral discontinuities between the F450W and F300W passbands, indicative of the presence of the Lyman continuum break in the redshift range 2.4 < z < 3.4. We have employed color selection criteria similar to those we have used for selecting high redshift galaxy candidates from ground--based images. We find that, as for the ground--based color selection, the criteria are very successful in selecting high redshift objects. Of the 8 galaxies observed (selected from a list of 23 candidates with magnitudes equivalent to R <= 25.3, five have confirmed redshifts in the range 2.59 <= z <= 3.22, with the remaining 3 being indeterminate because of contamination from nearby brighter objects. As expected, the HST filter system is sensitive to a somewhat broader range of redshifts than our ground--based U_nGR filter system, and therefore the surveyed volume per unit area on the sky is correspondingly larger. The distribution of candidates on the plane of the sky is clearly non--uniform, consistent with the available ground--based data on the high redshift galaxies. Most Lyman break objects in the Hubble Deep Field exhibit a similar range of morphological properties to the z > 3 galaxies we have previously identified in other fields, characterized by very compact cores (some with multiple components) with half--light radii of 0.2-0.3 arcseconds, often surrounded by more diffuse and asymmetric ``halos''. A few of the brighter HDF Lyman break galaxies, however, have particularly unusual morphologies.Comment: Accepted for publication in the Astronomical Journal. 18 pages including 6 postscript figures. Uses AAStex macros; aaspp4.sty include

We present results of our large area survey for z -band dropout galaxies at z = 7 in a 1568 arcmin 2 sky area covering the SDF and GOODS-N fields. Combining our ultra-deep Subaru/Suprime-Cam z -and y-band (λ eff = 1 μm) images with legacy data of Subaru and Hubble Space Telescope, we have identified 22 bright z-dropout galaxies down to y = 26, one of which has a spectroscopic redshift of z = 6.96 determined from Lyα emission. The z = 7 luminosity function yields the best-fit Schechter parameters of φ * = 0.69 +2.62 −0.55 × 10 −3 Mpc −3 , M * UV = −20.10 ± 0.76 mag, and α = −1.72 ± 0.65, and indicates a decrease from z = 6 at a > 95% confidence level. This decrease is beyond the cosmic variance in our two fields, which is estimated to be a factor of 2. We have found that the cosmic star formation rate density drops from the peak at z = 2-3 to z = 7 roughly by a factor of ∼10 but not larger than ∼100. A comparison with the reionization models suggests either that the universe could not be totally ionized by only galaxies at z = 7, or more likely that properties of galaxies at z = 7 are different from those at low redshifts having, e.g., a larger escape fraction ( 0.2), a lower metallicity, and/or a flatter initial mass function. Our SDF z-dropout galaxies appear to form 60 Mpc long filamentary structures, and the z = 6.96 galaxy with Lyα emission is located at the center of an overdense region consisting of four UV bright dropout candidates, which might suggest an existence of a well-developed ionized bubble at z = 7.