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

We have combined the results from our recent observations of damped and sub-damped Lyman alpha (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 N-HI-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 N-HI-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 N-HI 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 similar 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 similar to 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 Omega(met) similar to (3.5-15.8) x 10(5) M-circle dot 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.