We have discovered 16 Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to provide the first conclusive evidence for cosmic deceleration that preceded the current epoch of cosmic acceleration. These objects, discovered during the course of the GOODS ACS Treasury program, include 6 of the 7 highest-redshift SNe Ia known, all at z > 1.25, and populate the Hubble diagram in unexplored territory. The luminosity distances to these objects, and to 170 previously reported SNe Ia, have been determined using empirical relations between light-curve shape and luminosity. A purely kinematic interpretation of the SN Ia sample provides evidence at the > 99% confidence level for a transition from deceleration to acceleration or similarly, strong evidence for a cosmic jerk. Using a simple model of the -2expansion history, the transition between the two epochs is constrained to be at z = 0.46 ± 0.13. The data are consistent with the cosmic concordance model of Ω M ≈ 0.3, Ω Λ ≈ 0.7 (χ 2 dof = 1.06), and are inconsistent with a simple model of evolution or dust as an alternative to dark energy. For a flat Universe with a cosmological constant, we measure Ω M = 0.29± 0.05 0.03 (equivalently, Ω Λ = 0.71). When combined with external flat-Universe constraints including the cosmic microwave background and large-scale structure, we find w = −1.02± 0.13 0.19 (and w < −0.76 at the 95% confidence level) for an assumed static equation of state of dark energy, P = wρc 2 . Joint constraints on both the recent equation of state of dark energy, w 0 , and its time evolution, dw/dz, are a factor of ∼ 8 more precise than its first estimate and twice as precise as those without the SNe Ia discovered with HST. Our constraints are consistent with the static nature of and value of w expected for a cosmological constant (i.e., w 0 = −1.0, dw/dz = 0), and are inconsistent with very rapid evolution of dark energy. We address consequences of evolving dark energy for the fate of the Universe.

The Lyman decrement associated with the cumulative effect of H I in QSO absorption systems along the line of sight provides a distinctive feature for identifying galaxies at z ∼ > 2.5. Color criteria, which are sensitive to the presence of a Lyman-continuum break superposed on an otherwise flat UV spectrum, have been shown, through Keck spectroscopy, to successfully identify a substantial population of star-forming galaxies at 3 ∼ < z ∼ < 3.5 (Steidel et al. 1996a). Such objects have proven surprisingly elusive in fieldgalaxy redshift surveys; quantifying their surface density and morphology is crucial for determining how and when galaxies formed. The Hubble Deep Field (HDF) observations offer the opportunity to exploit the ubiquitous effect of intergalactic absorption and obtain useful statistical constraints on the redshift distribution of galaxies considerably fainter than current spectroscopic limits. We model the H I cosmic opacity as a function of redshift, including scattering in resonant lines of the Lyman series and Lymancontinuum absorption, and use stellar population synthesis models with a wide variety of ages, metallicities, dust contents, and redshifts, to derive color selection criteria that provide a robust separation between high redshift and low redshift galaxies. From the HDF images we construct a sample of star-forming galaxies at 2 ∼ < z ∼ < 4.5. While none of the ∼ 60 objects in the HDF having known Keck/LRIS spectroscopic redshifts in the range 0 ∼ < z ∼ < 1.4 is found to contaminate our high-redshift sample, our color criteria are able to efficiently select the 2.6 ∼ < z ∼ < 3.2 galaxies identified by Steidel et al. (1996b).The ultraviolet (and blue) dropout technique opens up the possibility of investigating cosmic star and element formation in the early universe. We set a lower-limit to the ejection rate of heavy elements per unit comoving volume from Type II supernovae at z = 2.75 of ≈ 3.6 × 10 −4 M ⊙ yr −1 Mpc −3 (for q 0 = 0.5 and H 0 = 50 km s −1 Mpc −1 ), which is 3 times higher than the local value, but still 4 times lower than the rate observed at z ≈ 1. At z = 4, our lower limit to the cosmic metal ejection rate is ≈ 3 times lower than the z = 2.75 value. We discuss the implications of these results on models of galaxy formation, and on the chemical enrichment and ionization history of the intergalactic medium.

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 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.