A GREEN BANK TELESCOPE SURVEY FOR H I 21 cm ABSORPTION IN THE DISKS AND HALOS OF LOW-REDSHIFT GALAXIES

Abstract

We present an H i 21 cm absorption survey with the Green Bank Telescope (GBT) of galaxy–quasar pairs selected by combining galaxy data from the Sloan Digital Sky Survey (SDSS) and radio sources from the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. Our sample consists of 23 sight lines through 15 low-redshift foreground galaxy–background quasar pairs with impact parameters ranging from 1.7 kpc up to 86.7 kpc. We detected one absorber in the GBT survey from the foreground dwarf galaxy, GQ1042+0747, at an impact parameter of 1.7 kpc and another possible absorber in our follow-up Very Large Array (VLA) imaging of the nearby foreground galaxy UGC 7408. The line widths of both absorbers are narrow (FWHM of 3.6 and 4.8km s−1). The absorbers have sub-damped Lyα column densities, and most likely originate in the disk gas of the foreground galaxies. We also detected H i emission from three foreground galaxies including UGC 7408. Although our sample contains both blue and red galaxies, the two H i absorbers as well as the H i emissions are associated with blue galaxies. We discuss the physical conditions in the 21 cm absorbers and some drawbacks of the large GBT beam for this type of survey.