A ground-based imaging study of galaxies causing damped Lyman α (DLA), sub-DLA and Lyman limit system absorption in quasar spectra

We present results from a search for galaxies that give rise to damped Lyman a (DLA), sub-DLA and Lyman limit system (LLS) absorption at redshifts 0.1 less than or similar to z less than or similar to 1 in the spectra of background quasars. The sample was formed from a larger sample of strong MgII absorbers (W-0(lambda 2796) >= 0.3 angstrom) whose HI column densities were determined by measuring the Ly alpha line in Hubble Space Telescope ultraviolet spectra. Photometric redshifts, galaxy colours and proximity to the quasar sightline, in decreasing order of importance, were used to identify galaxies responsible for the absorption. Our sample includes 80 absorption systems for which the absorbing galaxies have been identified, of which 54 are presented here for the first time. In some cases a reasonable identification for the absorbing galaxy could not be made. The main results of this study are (i) the surface density of galaxies falls off exponentially with increasing impact parameter, b, from the quasar sightline relative to a constant background of galaxies, with an e-folding length of approximate to 46 kpc. Galaxies with b greater than or similar to 100 kpc calculated at the absorption redshift are statistically consistent with being unrelated to the absorption system, and are either background or foreground galaxies. (ii) log N-HI is inversely correlated with b at the 3.0 sigma level of significance. DLA galaxies are found systematically closer to the quasar sightline, by a factor of 2, than are galaxies which give rise to sub-DLAs or LLSs. The median impact parameter is 17.4 kpc for the DLA galaxy sample, 33.3 kpc for the sub-DLA sample and 36.4 kpc for the LLS sample. We also find that the decline in log N-HI with b can be roughly described by an exponential with an e-folding length of 12 kpc that occurs at log N-HI = 20.0. (iii) Absorber galaxy luminosity relative to L*, L/L*, is not significantly correlated with W-0(lambda 2796), log N-HI or b. (iv) DLA, sub-DLA and LLS galaxies comprise a mix of spectral types, but are inferred to be predominantly late-type galaxies based on their spectral energy distributions. (v) The properties of low-redshift DLAs and sub-DLAs are very different in comparison to the properties of gas-rich galaxies at the present epoch. A significantly higher fraction of low-redshift absorbers have large b values, and a significantly higher fraction of the large b value galaxies have luminosities L < L*. The implications of these results are discussed.