A MIKE + UVES survey of sub-damped Lyman α systems at z < 1.5

Abstract

We have combined the results from our recent observations of damped and sub-damped Lyman α (sub-DLA) systems with the Magellan Inamori Kyocera Echelle and Ultraviolet Visual Echelle Spectrograph on the Magellan Clay and Very Large Telescopes with ones from the literature to determine the -weighted mean metallicity of these systems based both on Fe, a depleted element in quasi-stellar object absorbers and the local interstellar medium, and Zn a relatively undepleted element. In each case, the -weighted mean metallicity is higher and shows faster evolution in sub-DLAs than the classical DLA systems. Large grids of photoionization models over the sub-DLA range with cloudy show that the ionization corrections to the abundances are in general small, however the fraction of ionized H can be up to ∼90 per cent. The individual spectra have been shifted to the rest frame of the absorber and averaged together to determine the average properties of these systems at z < 1.5. We find that the average abundance pattern of the sub-DLA systems is similar to the gas in the halo of the Milky Way, with an offset of ∼0.3 dex in the overall metallicity. Both DLAs and sub-DLAs show similar characteristics in their relative abundances patterns, although the DLAs have smaller 〈[Mn/Zn]〉 as well as higher 〈[Ti/Zn]〉 and 〈[Cr/Zn]〉. We calculate the contribution of sub-DLAs to the metal budget of the Universe, and find that the sub-DLA systems at z < 1.5 contain a comoving density of metals Ωmet ∼ (3.5-15.8) × 105 M⊙ Mpc-3, at least twice the comoving density of metals in the DLA systems. The sub-DLAs do, however, track global chemical evolution models much more closely than do the DLAs, perhaps indicating that they are a less dust biased metallicity indicator of galaxies at high redshifts than the DLA systems. © 2009 The Authors. Journal compilation © 2009 RAS.