The chemical connection between damped Lyman-\alpha\ systems and Local Group dwarf galaxies

TL;DR

This study compares element abundances in dwarf galaxies and Damped Lyman-alpha systems, revealing differences in nucleosynthesis and star formation histories, and questioning the reliability of [S/Zn] as a proxy for alpha-element ratios.

Contribution

It demonstrates that [S/Zn] is not a reliable proxy for [$ ext{α}$/Fe] in Damped Lyman-alpha systems and proposes [O/S] as an alternative, highlighting differences in star formation histories.

Findings

[S/Zn] varies between systems and is unreliable as an alpha proxy.

Zn is not significantly produced by Type Ia supernovae.

Damped Lyman-alpha systems have different star formation histories from local dwarf galaxies.

Abstract

Abundances of the volatile elements S and Zn have now been measured in around 80 individual stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range $-2.4\leq\text{[Fe/H]}\leq-0.9$. These two elements are of particular interest as they are not depleted onto dust in gas, and their ratio, [S/Zn], has thus commonly been used as a proxy for [$\alpha$/Fe] in Damped Lyman-$\alpha$ systems. The S abundances in Sculptor are similar to other $\alpha$-elements in this galaxy, consistent with S being mainly created in core-collapse supernovae, but also having some contribution from supernovae Type Ia. However, our results show that Zn and Fe do not trace all the same nucleosynthetic production channels. In particular, (contrary to Fe) Zn is not significantly produced by supernovae Type Ia. Thus, [S/Zn] cannot be reliably used as a proxy for [$\alpha$/Fe]. We propose [O/S] as a function of [S/H] as a possible alternative. At higher metallicities, the values of [S/Zn] measured in Damped Lyman-$\alpha$ systems are inconsistent with those in local dwarf galaxies, and are more compatible with the Milky Way disk. Low-metallicity Damped Lyman-$\alpha$ systems are, however, consistent with the most metal-poor stars in Local Group dwarf spheroidal galaxies. Assuming that the dust depletions of S and Zn are negligible, our comparison indicates that the star formation histories of Damped Lyman-$\alpha$ systems are on average different from both the Milky Way and the Sculptor dwarf spheroidal galaxy.