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 the basic data for a large ground-based spectroscopic survey for z $ 3 Lyman break galaxies (LBGs), photometrically selected using rest-UV colors from very deep images in 17 high Galactic latitude fields. The total survey covers an area of 0.38 deg 2 and includes 2347 photometrically selected candidate LBGs to an apparent R AB magnitude limit of 25.5. Approximately half of these objects have been observed spectroscopically using the Keck telescopes, yielding 940 redshifts with hzi ¼ 2:96 AE 0:29. We discuss the images, photometry, target selection, and spectroscopic program in some detail and present catalogs of the photometric and spectroscopic data, made available in electronic form. We discuss the general utility of conducting nearly volume-limited redshift surveys in prescribed redshift intervals using judicious application of photometric preselection.

We present the results of a systematic study of the rest-frame UV spectroscopic properties of Lyman break galaxies (LBGs). The database of almost 1000 LBG spectra proves useful for constructing high signalto-noise composite spectra. The composite spectrum of the entire sample reveals a wealth of features attributable to hot stars, H ii regions, dust, and outflowing neutral and ionized gas. By grouping the database according to galaxy parameters such as Ly equivalent width, UV spectral slope, and interstellar kinematics, we isolate some of the major trends in LBG spectra that are least compromised by selection effects. We find that LBGs with stronger Ly emission have bluer UV continua, weaker low-ionization interstellar absorption lines, smaller kinematic offsets between Ly and the interstellar absorption lines, and lower star formation rates. There is a decoupling between the dependence of low-and high-ionization outflow features on other spectral properties. Additionally, galaxies with rest-frame W Ly ! 20 Å in emission have weaker than average high-ionization lines and nebular emission lines that are significantly stronger than in the sample as a whole. Most of the above trends can be explained in terms of the properties of the large-scale outflows seen in LBGs. According to this scenario, the appearance of LBG spectra is determined by a combination of the covering fraction of outflowing neutral gas, which contains dust and the range of velocities over which this gas is absorbing. In contrast, the strengths of collisionally excited nebular emission lines should not be affected by the nature of the outflow, and variations in these lines may indicate differences in the temperatures and metallicities in H ii regions of galaxies with very strong Ly emission. Higher sensitivity and spectral resolution observations are still required for a full understanding of the covering fraction and velocity dispersion of the outflowing neutral gas in LBGs and its relationship to the escape fraction of Lyman continuum radiation in galaxies at z $ 3.

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We present initial results of a survey for star-forming galaxies in the redshift range This 3.8 [ z [ 4.5. sample consists of a photometric catalog of 244 galaxies culled from a total solid angle of 0.23 deg2 to an apparent magnitude of Spectroscopic redshifts in the range 3.61 ¹ z ¹ 4.81 have been I AB \ 25.0. obtained for 48 of these galaxies ; their median redshift is SzT \ 4.13. Selecting these galaxies in a manner entirely analogous to our large survey for Lyman-break galaxies at smaller redshift (2.7 [ z [ 3.4) allows a relatively clean di †erential comparison between the populations and integrated luminosity density at these two cosmic epochs. Over the same range of UV luminosity, the spectroscopic properties of the galaxy samples at z D 4 and z D 3 are indistinguishable, as are the luminosity function shapes and the total integrated UV luminosity densities We see no evidence at. these bright magnitudes for the steep decline in the star formation density inferred from fainter photometric Lyman-break galaxies in the Hubble deep Ðeld (HDF). The HDF provides the only existing data on Lyman-break galaxy number densities at fainter magnitudes. We have reanalyzed the z D 3 and z D 4 Lyman-break galaxies in the HDF using our improved knowledge of the spectral energy distributions of these galaxies, and we Ðnd, like previous authors, that faint Lyman-break galaxies appear to be rarer at z D 4 than z D 3. This might signal a large change in the faint-end slope of the Lyman-break galaxy luminosity function between redshifts z D 3 and z D 4, or, more likely, be due to signiÐcant variance in the number counts within the small volumes probed by the HDF at high redshifts (D160 times smaller than the ground-based surveys discussed here).If the true luminosity density at z D 4 is somewhat higher than implied by the HDF, as our groundbased sample suggests, then the emissivity of star formation as a function of redshift would appear essentially constant for all z [ 1 once internally consistent corrections for dust are made. This suggests that there may be no obvious peak in star formation activity and that the onset of substantial star formation in galaxies might occur at z Z 4.5.

We present the Ðrst results of a spectroscopic survey of Lyman break galaxies (LBGs) in the nearinfrared aimed at detecting the emission lines of [O II], [O III], and Hb from the H II regions of normal star-forming galaxies at z^3. From observations of 15 objects with the Keck telescope and the Very Large Telescope augmented with data from the literature for an additional four objects, we reach the following main conclusions. The rest-frame optical properties of LBGs at the bright end of the luminosity function are remarkably uniform, their spectra are dominated by emission lines, [O III] is always stronger than Hb and [O II], and projected velocity dispersions are between 50 and 115 km s~1. Contrary to expectations, the star formation rates deduced from the Hb luminosity are on average no larger than those implied by the stellar continuum at 1500 presumably any di †erential extinction between A ; rest-frame optical and UV wavelengths is small compared to the relative uncertainties in the calibrations of these two star formation tracers. For the galaxies in our sample, the abundance of oxygen can only be determined to within 1 order of magnitude without recourse to other emission lines ([N II] and Ha), which are generally not available. Even so, it seems well established that LBGs are the most metalenriched structures at z^3, apart from quasi-stellar objects, with abundances greater than about 1/10 solar and generally higher than those of damped Lya systems at the same epoch. They are also signiÐ-cantly overluminous for their metallicities ; this is probably an indication that their mass-to-light ratios are low compared to present-day galaxies. At face value, the measured velocity dispersions imply virial masses of about 1010 within half-light radii of 2.5 kpc. The corresponding mass-to-light ratios, M _ M/L B 0.15 in solar units, are indicative of stellar populations with ages between 108 and 109 yr, consistent with the UV-optical spectral energy distributions. However, we are unable to establish conclusively whether or not the widths of the emission lines reÑect the motions of the H II regions within the gravitational potential of the galaxies, even though in two cases we see hints of rotation curves. All 19 LBGs observed show evidence for galactic-scale superwinds ; such outÑows have important consequences for regulating star formation, distributing metals over large volumes, and allowing Lyman continuum photons to escape and ionize the intergalactic medium.

We present the results of a near-infrared imaging survey of z D 3 Lyman break galaxies (LBGs). The survey covers a total of 30 arcmin2 and includes 118 photometrically selected LBGs with mea-K s -band surements, 63 of which also have J-band measurements, and 81 of which have spectroscopic redshifts. Using the distribution of optical R magnitudes from previous work and colors for this sub-R[K s sample, we compute the rest-frame optical luminosity function of LBGs. This luminosity function is described by an analytic Schechter Ðt with a very steep faint-end slope of a \ [1.85^0.15, and it strikingly exceeds locally determined optical luminosity functions at brighter magnitudes, where it is fairly well constrained. The V -band luminosity density of only the observed bright end of the z D 3 LBG luminosity function already approaches that of all stars in the local universe. For the 81 galaxies with measured redshifts, we investigate the range of LBG stellar populations implied by the photometry that generally spans the range 900È5500 in the rest frame. The parameters under consideration are the star A formation rate as a function of time, the time since the onset of star formation, and the degree of reddening and extinction by dust. While there are only weak constraints on the parameters for most of the individual galaxies, there are strong trends in the sample as a whole. With a wider wavelength baseline than most previous studies at similar redshifts, we conÐrm the trend that intrinsically more luminous galaxies are dustier. We also Ðnd that there is a strong correlation between extinction and the age of the star formation episode, in the sense that younger galaxies are dustier and have much higher star formation rates. The strong correlation between extinction and age, which we show is unlikely to be an artifact of the modeling procedure, has important implications for an evolutionary sequence among LBGs. A uniÐed scenario that accounts for the observed trends in bright LBGs is one in which a relatively short period of very rapid star formation (hundreds of yr~1) lasts for roughly 50È100 Myr, after which M _ both the extinction and star formation rate are considerably reduced and stars are formed at a more quiescent, but still rapid, rate for at least a few hundred megayears. In our sample, a considerable fraction (D20%) of the LBGs have best-Ðt star formation ages Gyr, implied stellar masses of Z1 Z1010 and are still forming stars at D30 yr~1. M _ , M _

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.