Si and Mn abundances in damped Ly alpha systems with low dust content

We have measured the abundances of Zn, Si, Mn, Cr, Fe, and Ni in three damp ed Ly alpha systems at redshifts z(abs) less than or equal to 1 from high-r esolution echelle spectra of QSOs recorded with the Keek I telescope. In al l 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 refracto ry elements locked up in dust grains is less than about 50%, it is plausibl e to assume an approximately uniform level of depletion for all grain const ituents 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 s ystems (6-8 cases) for which it is appropriate, with the aim of comparing t he metallicity dependence of the ratios [Si/Fe] and [Mn/Fe] with analogous measurements in Milky Way stars. The main conclusion is that the relative a bundances of both elements in distant galaxies are broadly in line with exp ectations based on Galactic data. Si displays a mild enhancement at low met allicities, as expected for an a-capture element, but there are also exampl es of near-solar [Si/Fe] at [Fe/H] < -1. The underabundance of Mn at low me tallicities 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 hete rogeneous chemical properties of damped Ly alpha systems, evident even from this limited set of measurements, provide further support for the conclusi on 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 held of one of the QSOs, Q0058 + 019, showing the presence of an ed ge-on late-type galaxy only 1." 2 from the absorption sight line. If this i s the galaxy producing the damped Ly alpha system at z(abs) = 0.61251, it i s of relatively low luminosity (M-B = - 19.1) and at an impact parameter of 10 h(50)(-1) kpc.