Metal-rich damped/subdamped Lyman α quasar absorbers at z < 1*

Abstract

Damped Lyman α absorbers (DLAs), seen in absorption against a background quasar, provide the most detailed probes available of element abundances in the Universe over >90per cent of its age. DLAs can be used to observationally measure the global mean metallicity in the Universe and its evolution with time. Paradoxically, these observations are more difficult at lower redshifts, where the absorber rest-frame ultraviolet (UV) spectra are cut-off due to the atmospheric absorption. We present here high-resolution Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph observations of several elements contained in three DLAs and one sub-DLA with 0.6 < z abs < 0.9. We detect Mg I, Mg II, Fe II, Zn II, Cr II, Mn II, Ti II and Ca II. Our observations more than double the high-resolution sample of [Zn/H] at z < 1. We also report the discovery of three metal-rich systems, whereas most previous measurements show low AHI-weighted mean metallicity projecting to about l/6th solar level at z = 0. We derive [Zn/H] = -0.11 ± 0.04 at zabs = 0.725, [Zn/H] = -0.54 ± 0.20 at zabs = 0.740 and [Zn/H] = -0.49 ± 0.22 at zabs = 0.652, plus one additional upper limit ([Zn/H] < -0.36 at zabs = 0.842). These measurements confirm the existence of quasar absorbers with relatively high metallicities based on abundance estimates free from the effect of dust depletion. Possible implications of these results for the metallicity of neutral gas phase in the past ≈8 Gyr are presented and compared with models. © 2006 RAS.