Si and Mn Abundances in Damped Lyα Systems with Low Dust Content

Abstract

We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damped Lyα systems at redshifts zabs ≤ 1 from high-resolution echelle spectra of QSOs recorded with the Keck I telescope. In all three cases, the abundances of Cr, Fe, and Ni relative to Zn indicate low levels of dust depletions. We propose that when the proportion of refractory elements locked up in dust grains is less than about 50%, it is plausible to assume an approximately uniform level of depletion for all grain constituents and, by applying a small dust correction, to recover the intrinsic abundances of Si and Mn. We use this approach on a small sample of damped systems (6-8 cases) for which it is appropriate, with the aim of comparing the metallicity dependence of the ratios [Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main conclusion is that the relative abundances of both elements in distant galaxies are broadly in line with expectations based on Galactic data. Si displays a mild enhancement at low metallicities, as expected for an α-capture element, but there are also examples of near-solar [Si/Fe] at [Fe/H] < -1. The underabundance of Mn at low metallicities is possibly even more pronounced than in metal-poor stars, and no absorption system has yet been found in which [Mn/Fe] is solar. The heterogeneous chemical properties of damped Lyα systems, evident even from this limited set of measurements, provide further support for the conclusion from imaging studies that a varied population of galaxies gives rise to this class of QSO absorbers. We also present a Hubble Space Telescope image of the field of one of the QSOs, Q0058+019, showing the presence of an edge-on late-type galaxy only 12 from the absorption sight line. If this is the galaxy producing the damped Lyα system at zabs = 0.61251, it is of relatively low luminosity (MB = -19.1) and at an impact parameter of 10 h kpc.