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.

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.

We report subarcsecond resolution IRAM PdBI millimeter CO interferometry of four z $ 2 submillimeter galaxies (SMGs), and sensitive CO(3Y2) flux limits toward three z $ 2 UV/optically selected star-forming galaxies. The new data reveal for the first time spatially resolved CO gas kinematics in the observed SMGs. Two of the SMGs show double or multiple morphologies, with complex, disturbed gas motions. The other two SMGs exhibit CO velocity gradients of $500 km s À1 across 0.2 00 (1.6 kpc) diameter regions, suggesting that the star-forming gas is in compact, rotating disks. Our data provide compelling evidence that these SMGs represent extreme, short-lived ''maximum'' star-forming events in highly dissipative mergers of gas-rich galaxies. The resulting high-mass surface and volume densities of SMGs are similar to those of compact quiescent galaxies in the same redshift range and much higher than those in local spheroids. From the ratio of the comoving volume densities of SMGs and quiescent galaxies in the same mass and redshift ranges, and from the comparison of gas exhaustion timescales and stellar ages, we estimate that the SMG phase duration is about 100 Myr. Our analysis of SMGs and optically/ UV selected high-redshift starforming galaxies supports a ''universal'' Chabrier IMF as being valid over the star-forming history of these galaxies. We find that the 12 CO luminosity to total gas mass conversion factors at z $ 2Y3 are probably similar to those assumed at z $ 0. The implied gas fractions in our sample galaxies range from 20% to 50%.

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.

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z $ 2. By combining the H spectra with photometric measurements from observed 0.3-8 m, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The H line widths give a mean dynamical mass M dyn ¼ (6:9 AE 0:6) ; 10 10 M within a typical radius of $6 kpc, after excluding AGNs. The average dynamical mass is $2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, $15% of the sample has M dyn 3 M ? . These objects are best fit by young stellar populations and tend to have high H equivalent widths, W H k 200 8, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of $50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of $500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of $40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.

We present the results of a spectroscopic survey of the kinematic structure of star-forming galaxies at redshift z ∼ 2 − 3 using Keck/OSIRIS integral field spectroscopy. Our sample is comprised of 12 galaxies between redshifts z ∼ 2.0 and 2.5 and one galaxy at z ∼ 3.3 which are well detected in either Hα or [O iii] emission. These galaxies are generally representative of the mean stellar mass of star forming galaxies at similar redshifts, although they tend to have star formation rate surface densities slightly higher than the mean. These observations were obtained in conjunction with the Keck laser guide star adaptive optics system, with a typical angular resolution after spatial smoothing ∼ 0.15" (approximately 1 kpc at the redshift of the target sample). At most five of these 13 galaxies have spatially resolved velocity gradients consistent with rotation while the remaining galaxies have relatively featureless or irregular velocity fields. All of our galaxies show local velocity dispersions ∼ 60 -100 km s −1 , suggesting that (particularly for those galaxies with featureless velocity fields) rotation about a preferred axis may not be the dominant mechanism of physical support. While some galaxies show evidence for major mergers such evidence is unrelated to the kinematics of individual components (one of our strongest merger candidates also exhibits unambiguous rotational structure), refuting a simple bimodal disk/merger classification scheme. We discuss these data in light of complementary surveys and extant UV-IR spectroscopy and photometry, concluding that the dynamical importance of cold gas may be the primary factor governing the observed kinematics of z ∼ 2 galaxies. We conclude by speculating on the importance of mechanisms for accreting low angular-momentum gas and the early formation of quasi-spheroidal systems in the young universe.

We use a sample of 90 spectroscopically-confirmed Lyman Break Galaxies with Hα measurements and Spitzer MIPS 24 µm observations to constrain the relationship between rest-frame 8 µm luminosity (L 8 ) and star formation rate (SFR) for L * galaxies at z ∼ 2. We find a tight correlation with 0.24 dex scatter between L 8 and Hα luminosity/SFR for z ∼ 2 galaxies with 10 10 L IR 10 12 L ⊙ . Employing this relationship with a larger sample of 392 galaxies with spectroscopic redshifts, we find that the UV slope β can be used to recover the dust attenuation of the vast majority of moderately luminous L * galaxies at z ∼ 2 to within a 0.4 dex scatter using the local correlation. Separately, young galaxies with ages 100 Myr appear to be less dusty than their UV slopes would imply based on the local trend and may follow an extinction curve that is steeper than what is typically assumed. Consequently, very young galaxies at high redshift may be significantly less dusty than inferred previously. Our results provide the first direct evidence, independent of the UV slope, for a correlation between UV and bolometric luminosity (L bol ) at high redshift, in the sense the UV-faint galaxies are also on average less infrared and less bolometrically-luminous than their UV-bright counterparts. The L bol -L UV relation indicates that as the SFR increases, L UV turns over (or "saturates") around the value of L * at z ∼ 2, implying that dust obscuration may be largely responsible for modulating the bright-end of the UV luminosity function. Finally, dust attenuation is found to correlate with oxygen abundance at z ∼ 2. Accounting for systematic differences in local and high-redshift metallicity determinations, we find that L * galaxies at z ∼ 2, while at least an order of magnitude more bolometrically-luminous, exhibit ratios of metals-to-dust that are similar to those of local starbursts. This result is expected if high-redshift galaxies are forming their stars in a less metal-rich environment compared to local galaxies of the same luminosity, thus naturally leading to a redshift evolution in both the bolometric luminosity -metallicity and bolometric luminosity -obscuration relations.