H I 21 Centimeter Absorption in Two Low-Redshift Damped Lyα Systems

We report the discovery of two low-redshift H I 21 cm absorbers, one at z = 0.2212 toward the zem = 0.630 quasar OI 363 (B0738+313), and the other at z = 0.3127 toward PKS B1127-145 (zem = 1.187). Both were found during a survey of Mg II–selected systems at redshifts 0.2 < z < 1 using the new UHF-high system at the Westerbork Synthesis Radio Telescope. New Hubble Space Telescope Faint Object Spectrograph observations also identify both systems as damped Lyα (DLA) absorbers. By comparing the column density from the DLA line with that from the H I 21 cm line, we calculate the spin temperature, Ts, of the two systems. We find Ts ≈ 1000 K for both these low-redshift absorbers. For the z = 0.3127 system toward PKS B1127-145, two galaxies have been previously identified with emission lines at the absorber redshift by Bergeron &Boissé, with the galaxy at a closer projected distance to the quasar assumed to be responsible for the absorption system. An ESO New Technology Telescope/EFOSC2 spectrum of a third, fainter companion at 3.″9, or 11 h kpc, from the line of sight of PKS B1127-145 reveals [O III] λ4958 and λ5007 at z = 0.3121 ± 0.0003. We consider this object the most likely to be responsible for the 21 cm absorption, as it is much closer to the QSO sight line than the two galaxies identified by Bergeron &Boissé.