Abundances at High Redshifts: The Chemical Enrichment History OF

Abstract

We study the elemental abundances of C, N, O, Al, Si, S, Cr, Mn, Fe, Ni, and Zn in a sample of 14 damped Lyα systems (galaxies) with H I column density N (H I)≥ 10 20 cm −2 , using high quality spectra of quasars obtained with the 10m Keck telescope. To ensure accuracy, only weak, unsaturated absorption lines are used to derive ion column densities and elemental abundances. Combining these abundance measurements with similar measurements in the literature, we investigate the chemical evolution of damped Lyα galaxies based on a sample of 23 systems in the redshift range 0.7 < z < 4.4. The main conclusions are as follows.1. The damped Lyα galaxies have (Fe/H) in the range of 1/10 to 1/300 solar, clearly indicating that these are young galaxies in the early stages of chemical evolution. The N (H I)-weighted mean metallicity of the damped Lyα galaxies between 2 < z < 3 is (Fe/H)=0.028 solar. There is a large scatter, about a factor of 30, in (Fe/H) at z < 3, which we argue probably results from the different formation histories of the absorbing galaxies or a mix of galaxy types.2. Comparisons of the distribution of (Fe/H) vs redshift for the sample of damped Lyα galaxies with the similar relation for the Milky Way disk indicate that the damped Lyα galaxies are much less metal-enriched than the Galactic disk in its past. Since there is evidence from our analyses that depletion of Fe by dust grains in the sample galaxies is relatively unimportant, the difference in the enrichment level between the sample of damped Lyα galaxies and the Milky Way disk suggests that damped Lyα galaxies are probably not high-redshift spiral disks in the traditional sense. Rather, they could represent a thick disk phase of galaxies, or more likely the spheroidal component of galaxies, or dwarf galaxies.3. The mean metallicity of the damped Lyα galaxies is found to increase with decreasing redshift, as is expected. All four of the damped Lyα galaxies at z > 3 in our sample have (Fe/H) around 1/100 solar or less. In comparison, a large fraction of the damped Lyα galaxies at z < 3 have reached ten times higher metallicity. This suggests that the time around z = 3 may be the epoch of galaxy formation in the sense that galaxies are beginning to form the bulk of their stars. Several other lines of evidence appear to point to the same conclusion, including the evolution of the neutral baryon 1 Hubble Fellow

The Chemical Evolution of Light Elements in Our Galaxy

“…It may well be indeed, that most of the lines of sight to quasars intercept various parts of (proto)galactic disks. This conclusion is opposite to the one reached in [15], which was based on a comparison of the data to the age-metallicity relationship of the LD only. Notice that for geometrical reasons, the probability of detecting outer disk regions is higher, favouring systematically lower abundances than those spanned during the LD history.…”

“…should account for the discrepancy between theory (solid line) and observations (data from [11]) b) Evolution of gas and stars; data for HI gas density from [29]. c) The evolution of Zn/H in various regions of spiral disks (only three are shown here, corresponding to those on the left) brackets well the observed abundances of Zn/H in Lyo absorbers (data from [18,15]); those systems may well be (proto)galactic disks. d) The corresponding evolution of D shows that considerable depletion may take place in the inner disk regions, but only at low redshifts; abundances measured at high redshifts should be close to the primordial value.…”

“…These observations concern: i) the "global" SFR ( [11]); ii) the HI content of the Universe ( [29]) and iii) the abundances of various elements in gaseous systems, in the line of sight of quasars ( [15,18]). To these one should include observations of D abundances, although the current status of those observations does not allow to establish a trend as a function of redshift.…”

On the Cosmic Origins of Carbon and Nitrogen

“…However, it is also clear that stars less massive than about 5M produce and expel carbon as well (van den Hoek & Groenewegen 1997;Marigo et al 1996Marigo et al , 1998Henry et al 2000), and the relative significance of massive and intermediate mass stars has not been established. Lu et al (1996; and with the observed abundances of blue compact galaxies found by Izotov & Thuan (1999) at very low metallicity. Then, at 12+log(O/H)>8 N/O turns upward and rises steeply.…”

“…Filled circles are stellar data from . The circle indicates the position of the Orion Nebula (Esteban et al 1998), the large S shows the position of the sun (Grevesse et al 1996), and the L symbols at extremely low oxygen show upper limits for two high redshift damped Lyman-α objects in Lu et al (1996). Progenitor star metallicities are identified by line types, as indicated in the legend.…”