Abundance Profiles and Kinematics of Damped Lyα Absorbing Galaxies at z < 0.65

Abstract

We present a spectroscopic study of six damped Lyα absorption (DLA) systems at z<0.65, based on moderate- to high-resolution spectra of the galaxies responsible for the absorbers. Combining known metallicity measurements of the absorbers with known optical properties of the absorbing galaxies, we confirm that the low metal content of the DLA population can arise naturally as a combination of gas cross section selection and the metallicity gradients commonly observed in local disk galaxies. We also study the Tully-Fisher relation of the DLA-selected galaxies and find little detectable evidence for evolution in the disk population between z=0 and ~0.5. Additional results of our analysis are as follows: (1) The DLA galaxies exhibit a range of spectral properties, from poststarburst, to normal disks, to starburst systems, supporting the idea that DLA galaxies are drawn from the typical field population. (2) Large rotating H I disks of radius 30 h-1 kpc and of dynamic mass Mdyn>1011 h-1 Msolar appear to be common at intermediate redshifts. (3) Using an ensemble of six galaxy-DLA pairs, we derive an abundance profile that is characterized by a radial gradient of -0.041+/-0.012 dex kpc-1 (or, equivalently, a scale length of 10.6 h-1 kpc) from the galactic center to a 30 h-1 kpc radius. (4) Adopting known N(H I) profiles of nearby galaxies and the best-fit radial gradient, we further derive an N(H I)-weighted mean metallicity N(HI)=-0.50+/-0.07 for the DLA population over 100 random lines of sight, consistent with N(HI)=-0.64+0.40-0.86 observed for z~1 DLA systems from Prochaska et al. Our analysis demonstrates that the low metal content of DLA systems does not rule out the possibility that the DLA population traces the field galaxy population. Observations reported here were obtained in part with the Magellan telescopes, a collaboration between the Observatories of the Carnegie Institution of Washington, University of Arizona, Harvard University, University of Michigan, and Massachusetts Institute of Technology. Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.