The Nature of Damped Lyα Absorbing Galaxies atz≤ 1: A Photometric Redshift Survey of Damped Lyα Absorbers

Abstract

We study the nature of damped Lyα absorption (DLA) systems at z ≤ 1 using a sample of 11 DLA galaxies, for which accurate redshift measurements are available. Five of the 11 systems are identified in our ongoing photometric redshift survey of DLA galaxies, while the remaining six systems are identified by previous groups using either spectroscopic or photometric redshift techniques. Absolute B-band magnitudes of the galaxies range from MAB(B) = -15.3 to -20.3. Impact parameter separations of the galaxy and absorber pairs range from ρ = 0.31 to 25.4 h-1 kpc. We first demonstrate that the precision of photometric redshifts is sufficient for identifying DLA galaxies because DLA systems are rare, and their intrinsically high column density implies a small impact parameter of the host galaxy to the QSO line of sight. We then adopt this first large DLA galaxy sample to study the neutral gas cross section of intermediate-redshift galaxies and examine the optical properties of DLA galaxies at z ≤ 1. The results of our study are as follows: (1) the extent of neutral gas around intermediate-redshift galaxies scales with B-band luminosity as R/R* = β with R* = 24-30 h-1 kpc and β = 0.26 at N(H ) = 1020 cm-2; (2) the observed incidence of the DLA systems versus the B-band luminosity of the DLA galaxies is consistent with models derived from adopting a known galaxy B-band luminosity function and the best-fit scaling relation of the neutral gas cross section at MAB(B) - 5 log h ≤ -17; (3) comparison of the observed and predicted number density of DLA systems supports that luminous galaxies can explain most of the DLA systems found in QSO absorption line surveys and a large contribution of dwarfs (MAB(B) - 5 log h ≥ -17) to the total neutral gas cross section is not necessary; (4) of the 11 DLA systems studied, 45% are disk dominated, 22% are bulge dominated, 11% are irregular, and 22% are in galaxy groups, indicating that galaxies that give rise to the DLA systems span a wide range of morphological types and arise in a variety of galaxy environments; and (5) galaxies that contain the bulk of neutral gas in the universe do not appear to exhibit a substantial luminosity evolution between z = 0 and 1.