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A GBT Survey for HI 21 cm Absorption in the Disks and Halos of Low-Redshift Galaxies

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