We present accurate metallicity measurements for 121 damped Lyα systems at 0.5 < z < 5 including ≈ 50 new measurements from our recently published Echellette Spectrograph and Imager surveys. This dataset is analysed to determine the age-metallicity relation of neutral gas in the universe. Contrary to previous datasets this sample shows statistically significant evolution in the mean metallicity. The best linear fit rate to metallicity vs. redshift is −0.26 ± 0.06 dex corresponding to approximately a factor of 2 every Gyr at z = 3. The DLA continue to maintain a floor in metallicity of ≈ 1/700 solar independent of observational effects. This metallicity threshold limits the prevalence of primordial gas in high redshift galaxies and stresses the correspondence between damped systems and star formation (i.e. galaxy formation). This floor is significantly offset from the metallicity of the Lyα forest and therefore we consider it to be more related to active star formation within these galaxies than scenarios of enrichment in the very early universe. Finally, we comment on an apparent 'missing metals problem': the mean metallicity of the damped systems is ≈ 10× lower than the value expected from their observed star formation history. This problem is evident in current theoretical treatments of chemical evolution and galaxy formation; it may indicate a serious flaw in our understanding of the interplay between star formation and metal production.8 This effect is less severe at z > 4 where there are fewer DLA with N (HI) > 10 21 cm −2 (see Péroux et al. 2001)

The SIRTF Wide-area InfraRed Extragalactic survey (SWIRE), the largest SIRTF Legacy program, is a wide-area, imaging survey to trace the evolution of dusty, starforming galaxies, evolved stellar populations, and AGN as a function of environment, from redshifts z∼3 to the current epoch. SWIRE will survey 7 high-latitude fields, totaling 60-65 sq. deg. in all 7 SIRTF bands: IRAC 3.6, 4.5, 5.6, 8µm and MIPS 24, 70, 160µm. Extensive modeling suggests that the Legacy Extragalactic Catalog may contain in excess of 2 million IR-selected galaxies, dominated by (1) ∼150,000 luminous infrared galaxies (LIRGs: L F IR > 10 11 L ⊙) detected by MIPS (and significantly more detected by IRAC), ∼7,000 of these with z>2; (2) 1 million IRAC-detected early-type galaxies (∼ 2×10 5 with z > 1 and ∼10,000 with z > 2); and (3) ∼ 20,000 classical AGN detected with MIPS, plus significantly more dust-obscured QSO/AGN among the LIRGs. SWIRE will provide an unprecedented view of the evolution of galaxies, structure, and AGN. The key scientific goals of SWIRE are: (1) to determine the evolution of actively star-forming and passively evolving galaxies in order to understand the history of galaxy formation in the context of cosmic structure formation; (2) to determine the evolution of the spatial distribution and clustering of evolved galaxies, starbursts and AGN in the key redshift range, 0.5

Discovery of the cosmic reionization epoch would represent a significant milestone in cosmology. We present Keck spectroscopy of the quasar SDSS 1044-0125, at z = 5.73. The spectrum shows a dramatic increase in the optical depth at observed wavelengths lambda >~7550 A, corresponding to z_abs >~ 5.2. Only a few small, narrow transmission regions are present in the spectrum beyond that point, and out to the redshifts where the quasar signal begins. We interpret this result as a signature of the trailing edge of the cosmic reionization epoch, which we estimate to occur around ~ 6 (as indeed confirmed by subsequent observations by Becker et al.), and extending down to z \~ 5.2. This behavior is expected in the modern theoretical models of the reionization era, which predict a patchy and gradual onset of reionization. The remaining transmission windows we see may correspond to the individual reionization bubbles (Stromgren spheres) embedded in a still largely neutral intergalactic medium, intersected by the line of sight to the quasar. Future spectroscopic observations of quasars at comparable or larger redshifts will provide a more detailed insight into the structure and extent of the reionization era.Comment: Replaced with the revised version. To appear in The Astrophysical Journal Letters. Latex file, 13 pages, 3 eps figures included, AASTEX style files include