Estimating the mass density of neutral gas at z<1z < 1

We use the relationships between galactic HI mass and B-band luminosity determined by Rao & Briggs to recalculate the mass density of neutral gas at the present epoch based on more recent measures of the galaxy luminosity function than were available to those authors. We find $\Omega_{gas}(z=0) \simeq 5 \times 10^{-4}$ in good agreement with the original Rao & Briggs value, suggesting that this quantity is now reasonably secure. We then show that, if the scaling between H I mass and B-band luminosity has remained approximately constant since $z = 1$, the evolution of the luminosity function found by the Canada-France redshift survey translates to an increase of $\Omega_{gas}$ by a factor of $\approx 3$ at $z = 0.5 - 1$ . A similar value is obtained quite independently from consideration of the luminosity function of Mg II absorbers at $z = 0.65$. By combining these new estimates with data from damped \lya systems at higher redshift, it is possible to assemble a rough sketch of the evolution of $\Omega_{gas}$ over the last 90% of the age of the universe. The consumption of H I gas with time is in broad agreement with models of chemical evolution which include the effects of dust, although more extensive samples of damped \lya systems at low and intermediate redshift are required for a quantitative assessment of the dust bias.Comment: LaTeX file, 11 pages, 1 figure, accepted MNRAS pink page

Near-pristine gas at high redshifts: a window on early nucleosynthesis

It has now become recognised that damped Lyman alpha systems play an important role in helping us unravel the origin of chemical elements. In this presentation, we describe the main results of a recently completed survey of the most metal-poor DLAs, aimed at complementing and extending studies of the oldest stars in the Galaxy. The survey has clarified a number of lingering issues concerning the abundances of C, N, O in the low metallicity regime, has revealed the existence of DLA analogues to Carbon-enhanced metal-poor stars, and is providing some of the most precise measures of the primordial abundance of Deuterium.Comment: 11 pages, 7 Figures. Invited presentation at the XII International Symposium on Nuclei in the Cosmos, Cairns, Australia, 5-10 August 2012. To appear in Proceedings of Scienc

The star formation rate of CaII and damped Lyman-alpha absorbers at 0.4<z<1.3

[abridged] Using stacked Sloan Digital Sky Survey spectra, we present the detection of [OII]3727,3730 nebular emission from galaxies hosting CaII and MgII absorption line systems. Both samples of absorbers, 345 CaII systems and 3461 MgII systems, span the redshift interval 0.4 < z < 1.3; all of the former and half the latter sample are expected to be bona-fide damped Lyman-alpha (DLA) absorbers. The measured star formation rate (SFR) per absorber from light falling within the SDSS fibre apertures (corresponding to physical radii of 6-9 h^-1 kpc) is 0.11-0.14 Msol/yr for the MgII-selected DLAs and 0.11-0.48 Msol/yr for the CaII absorbers. These results represent the first estimates of the average SFR in an absorption-selected galaxy population from the direct detection of nebular emission. Adopting the currently favoured model in which DLAs are large, with radii >9h^-1 kpc, and assuming no attenuation by dust, leads to the conclusion that the SFR per unit area of MgII-selected DLAs falls an order of magnitude below the predictions of the Schmidt law, which relates the SFR to the HI column density at z~0. The contribution of both DLA and CaII absorbers to the total observed star formation rate density in the redshift range 0.4 < z < 1.3, is small, <10% and <3% respectively. The result contrasts with the conclusions of Hopkins et al. that DLA absorbers can account for the majority of the total observed SFR density in the same redshift range. Our results effectively rule out a picture in which DLA absorbers are the sites in which a large fraction of the total SFR density at redshifts z < 1 occurs.Comment: Accepted for publication in MNRAS, 13 pages, 6 figure

Mixing Metals in the Early Universe

We investigate the evolution of the metallicity of the intergalactic medium (IGM) with particular emphasis on its spatial distribution. We propose that metal enrichment occurs as a two step process. First, supernova (SN) explosions eject metals into relatively small regions confined to the surroundings of star-forming galaxies. From a comprehensive treatment of blowout we show that SNae by themselves fail by more than one order of magnitude to distribute the products of stellar nucleosynthesis over volumes large enough to pollute the whole IGM to the metallicity levels observed. Thus, a additional (but as yet unknown) physical mechanism must be invoked to mix the metals on scales comparable to the mean distance between the galaxies which are most efficient pollutants. From this simple hypothesis we derive a number of testable predictions for the evolution of the IGM metallicity. Specifically, we find that: (i) the fraction of metals ejected over the star formation history of the universe is about 50% at z=0; that is, approximately half of the metals today are found in the IGM; (ii) if the ejected metals were homogeneously mixed with the baryons in the universe, the average IGM metallicity would be ~ 1/25 Z(solar) at z=3. However, due to spatial inhomogeneities, the mean of the distribution of metallicities in the diffusive zones has a wide (more than 2 orders of magnitude) spread around this value; (iii) if metals become more uniformly distributed at z < 1, as assumed, at z = 0 the metallicity of the IGM is narrowly confined within the range Z ~ 0.1 +/- 0.03 Z(solar). Finally, we point out that our results can account for the observed metal content of the intracluster medium.Comment: 26 pages, LaTeX, 8 figures, accepted by MNRA