We use a sample of 87 rest-frame ultraviolet-selected star-forming galaxies with mean spectroscopic redshift z = 2.26±0.17 to study the correlation between metallicity and stellar mass at high redshift.Using stellar masses determined from spectral energy distribution fitting to U n GRJK s (and Spitzer IRAC, for 37% of the sample) photometry, we divide the sample into six bins in stellar mass, and construct six composite Hα + [N II] spectra from all of the objects in each bin. We estimate the mean oxygen abundance in each bin from the [N II]/Hα ratio, and find a monotonic increase in metallicity with increasing stellar mass, from 12 + log(O/H) < 8.2 for galaxies with M ⋆ = 2.7 × 10 9 M ⊙ to 12 + log(O/H) = 8.6 for galaxies with M ⋆ = 1.0 × 10 11 M ⊙ . The mass-metallicity relation at z ∼ 2 is offset from the local mass-metallicity relation by ∼ 0.3 dex, in the sense that galaxies of a given stellar mass have lower metallicity at high redshift. A corresponding metallicity-luminosity relation constructed by binning the galaxies according to rest-frame B magnitude shows no significant correlation. This lack of correlation is explained by the known large variation in the rest-frame optical mass-to-light ratio at z ∼ 2, and indicates that the correlation with stellar mass is more fundamental. We use the empirical relation between star formation rate density and gas density to estimate the gas fractions of the galaxies, finding an increase in gas fraction with decreasing stellar mass. The median gas fraction is more than two times higher than that found in local star-forming galaxies, providing a natural explanation for the lower metallicities of the z ∼ 2 galaxies. These gas fractions combined with the observed metallicities allow the estimation of the effective yield y eff as a function of stellar mass; in contrast to observations in the local universe which show a decrease in y eff with decreasing baryonic mass, we find a slight increase. Such a variation of metallicity with gas fraction is best fit by a model with supersolar yield and an outflow rate ∼ 4 times higher than the star formation rate. We conclude that the mass-metallicity relation at high redshift is driven by the increase in metallicity as the gas fraction decreases through star formation, and is likely modulated by metal loss from strong outflows in galaxies of all masses. Our ability to detect differential metal loss as a function of mass is limited by the small range of baryonic masses spanned by the galaxies in the sample, but there is no evidence for preferential loss of metals from low mass galaxies as has been suggested in the local universe.

We present new results on the kinematics and spatial distribution of metal-enriched gas within ∼ 125 kpc of star-forming ("Lyman Break") galaxies at redshifts 2 < ∼ z < ∼ 3. In particular, we focus on constraints provided by the rest-frame far-UV spectra of faint galaxies-and demonstrate how galaxy spectra can be used to obtain key spatial and spectral information more efficiently than possible with QSO sightlines. Using a sample of 89 galaxies with z = 2.3 ± 0.3 and with both rest-frame far-UV and Hα spectra, we re-calibrate the measurement of accurate galaxy systemic redshifts using only survey-quality rest-UV spectra. We use the velocity-calibrated sample to investigate the kinematics of the galaxy-scale outflows via the strong interstellar (IS) absorption lines and Lyman α emission (when present), as well as their dependence on other physical properties of the galaxies. We construct a sample of 512 close (1 − 15 ′′ ) angular pairs of z ∼ 2 − 3 galaxies with redshift differences indicating a lack of physical association. Sightlines to the background galaxies provide new information on the spatial distribution of circumgalactic gas surrounding the foreground galaxies. The close pairs sample galactocentric impact parameters 3-125 kpc (physical) at z = 2.2, providing for the first time a robust map of cool gas as a function of galactocentric distance for a well-characterized population of galaxies. We propose a simple model of circumgalactic gas that simultaneously matches the kinematics, depth, and profile shape of IS absorption and Lyα emission lines, as well as the observed variation of absorption line strength (H I and several metallic species) versus galactocentric impact parameter. Within the model, cool gas is distributed symmetrically around every galaxy, accelerating radially outward with v out (r) increasing with r (i.e., the highest velocities are located at the largest galactocentric distances r). The inferred radial dependence of the covering fraction of cool gas (which modulates the absorption line strength) is f c (r) ∝ r −γ with 0.2 < ∼ γ < ∼ 0.6 depending on transition. We discuss the results of the observations in the context of "cold accretion", in which cool gas is accreting via filamentary streams directly onto the central regions of galaxies. At present, we find little observational evidence for cool infalling material, while evidence supporting the large-scale effects of superwind outflows is strong. This "pilot" study using faint galaxy spectra demonstrates the potential of using galaxies to trace baryons within galaxies, in the circumgalactic medium, and ultimately throughout the IGM.

The radio counterparts to the IRAS Redshift Survey galaxies are identified in the NRAO VLA Sky Survey (NVSS) catalog. Our new catalog of the infrared flux-limited (S 60µm ≥ 2 Jy) complete sample of 1809 galaxies lists accurate radio positions, redshifts, and 1.4 GHz radio and IRAS fluxes. This sample is six times larger in size and five times deeper in redshift coverage (to z ≈ 0.15) compared with those used in earlier studies of the radio and far-infrared (FIR) properties of galaxies in the local volume. The well known radio-FIR correlation is obeyed by the overwhelming majority (≥ 98%) of the infrared-selected galaxies, and the radio AGNs identified by their excess radio emission constitute only about 1% of the sample, independent of the IR luminosity. These FIR-selected galaxies can account for the entire population of late-type field galaxies in the local volume, and their radio continuum may be used directly to infer the extinction-free star formation rate in most cases. Both the 1.4 GHz radio and 60 µm infrared luminosity functions are reasonably well described by linear sums of two Schechter functions, one representing normal, late-type field galaxies and the second representing starbursts and other luminous infrared galaxies. The integrated FIR luminosity density for the local volume is (4.8 ± 0.5) × 10 7 L ⊙ Mpc −3 , less than 10% of which is contributed by the luminous infrared galaxies with L F IR ≥ 10 11 L ⊙ . The inferred extinction-free star formation density for the local volume is 0.015 ± 0.005 M ⊙ yr −1 Mpc −3 .

The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8-1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z > 1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10 9 M to z ≈ 2, reaching the knee of the ultraviolet luminosity function of galaxies to z ≈ 8. The survey covers approximately 800 arcmin 2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H = 27.7 mag) covers ∼125 arcmin 2 within Great Observatories Origins Deep Survey (GOODS)-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-deep Survey) and covers the full area to a 5σ pointsource limit of H 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding-cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey (KBSS) using MOSFIRE on the Keck 1 telescope, focusing on a sample of 251 galaxies with redshifts 2.0 < z < 2.6, star-formation rates 2 < ∼ SFR < ∼ 200 M yr −1 , and stellar masses 8.6 < log(M * /M ) < 11.4, with high-quality spectra in both H-and K-band atmospheric windows. We show unambiguously that the locus of z ∼ 2.3 galaxies in the "BPT" nebular diagnostic diagram exhibits a disjoint, yet similarly tight, relationship between the ratios [NII]λ6585/Hα and [OIII]/Hβ as compared to local galaxies. Using photoionization models, we argue that the offset of the z ∼ 2.3 locus relative to z ∼ 0 is explained by a combination of harder ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H than applies to most local galaxies, and that the position of a galaxy along the z ∼ 2.3 star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. The observed nebular emission line ratios are most easily reproduced by models in which the net ionizing radiation field resembles a blackbody with effective temperature T eff = 50000 − 60000 K and N/O close to the solar value at all O/H. We critically assess the applicability of commonly-used strong line indices for estimating gas-phase metallicities, and consider the implications of the small intrinsic scatter in the empirical relationship between excitation-sensitive line indices and M * (i.e., the "mass-metallicity" relation), at z 2.3.

This paper describes the Hubble Space Telescope imaging data products and data reduction procedures for the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). This survey is designed to document the evolution of galaxies and black holes at z ∼ 1.5 − 8, and to study Type Ia SNe beyond z > 1.5. Five premier multi-wavelength sky regions are selected, each with extensive multiwavelength observations. The primary CANDELS data consist of imaging obtained in the Wide Field Camera 3 / infrared channel (WFC3/IR) and UVIS channel, along with the Advanced Camera for Surveys (ACS). The CANDELS/Deep survey covers ∼ 125 square arcminutes within GOODS-N and GOODS-S, while the remainder consists of the CANDELS/Wide survey, achieving a total of ∼ 800 square arcminutes across GOODS and three additional fields (EGS, COSMOS, and UDS). We summarize the observational aspects of the survey as motivated by the scientific goals and present a detailed description of the data reduction procedures and products from the survey. Our data reduction methods utilize the most up to date calibration files and image combination procedures. We have paid special attention to correcting a range of instrumental effects, including CTE degradation for ACS, removal of electronic bias-striping present in ACS data after SM4, and persistence effects and other artifacts in WFC3/IR. For each field, we release mosaics for individual epochs and eventual mosaics containing data from all epochs combined, to facilitate photometric variability studies and the deepest possible photometry. A more detailed overview of the science goals and observational design of the survey are presented in a companion paper.

We present the deepest 100 to 500 μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3-500 μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes. We find that the ratio of total IR luminosity to rest-frame 8 μm luminosity, IR8 (≡L tot IR /L 8 ), follows a Gaussian distribution centered on IR8 = 4 (σ = 1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population (<20%) is shown to consist of starbursts with compact projected star formation densities. IR8 can be used to separate galaxies with normal and extended modes of star formation from compact starbursts with high-IR8, high projected IR surface brightness (Σ IR > 3 × 10 10 L kpc −2 ) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 Å size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample. Locally, luminous and ultraluminous IR galaxies, (U)LIRGs (L tot IR ≥ 10 11 L ), are systematically in the starburst mode, whereas most distant (U)LIRGs form stars in the "normal" main sequence mode. This confusion between two modes of star formation is the cause of the so-called "mid-IR excess" population of galaxies found at z > 1.5 by previous studies. Main sequence galaxies have strong polycyclic aromatic hydrocarbon (PAH) emission line features, a broad far-IR bump resulting from a combination of dust temperatures (T dust ∼ 15-50 K), and an effective T dust ∼ 31 K, as derived from the peak wavelength of their infrared SED. Galaxies in the starburst regime instead exhibit weak PAH equivalent widths and a sharper far-IR bump with an effective T dust ∼ 40 K. Finally, we present evidence that the mid-to-far IR emission of X-ray active galactic nuclei (AGN) is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty starbursts. After correcting for the effect of starbursts on IR8, we identify new candidates for extremely obscured AGNs.

The redshift interval 1:4 P z P 2:5 has been described by some as the ''redshift desert'' because of historical difficulties in spectroscopically identifying galaxies in that range. In fact, galaxies can be found in large numbers with standard broadband color selection techniques coupled with follow-up spectroscopy with UV and bluesensitive spectrographs. In this paper we present the first results of a large-scale survey of such objects, carried out with the blue channel of the LRIS spectrograph (LRIS-B) on the Keck I Telescope. We introduce two samples of star-forming galaxies, ''BX'' galaxies at hzi ¼ 2:20 AE 0:32 and ''BM'' galaxies at hzi ¼ 1:70 AE 0:34. In seven survey fields we have spectroscopically confirmed 749 of the former and 114 of the latter. Interlopers (defined as objects at z < 1) account for less than 10% of the photometric candidates, and the fraction of faint active galactic nuclei is $3% in the combined BX/BM sample. Deep near-IR photometry of a subset of the BX sample indicates that, compared with a sample of similarly UV-selected galaxies at z $ 3, the z $ 2 galaxies are on average significantly redder in (RÀK s ), indicating longer star formation histories, increased reddening by dust, or both. Using near-IR H spectra of a subset of BX/BM galaxies to define the galaxies' systemic redshifts, we show that the galactic-scale winds that are a feature of star-forming galaxies at z $ 3 are also common at later epochs and have similar bulk outflow speeds of 200-300 km s À1 . We illustrate with examples the information that can be deduced on the stellar populations, metallicities, and kinematics of redshift desert galaxies from easily accessible rest-frame far-UV and rest-frame optical spectra. Far from being hostile to observations, the universe at z $ 2 is uniquely suited to providing information on the astrophysics of star-forming galaxies and the intergalactic medium, and the relationship between the two.