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

TL;DR

This survey used the GBT to detect HI 21 cm absorption in low-redshift galaxy disks and halos, revealing narrow absorbers mainly associated with blue galaxies and highlighting the survey's limitations due to the GBT's large beam size.

Contribution

First systematic GBT survey of HI 21 cm absorption in low-redshift galaxy disks and halos, identifying absorbers and discussing physical conditions and observational challenges.

Findings

Detected two HI absorbers likely originating in galaxy disks.

Absorbers are narrow and associated with blue galaxies.

Detected HI emission in three foreground galaxies.

Abstract

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 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. Both of the absorbers are narrow (FWHM of 3.6 and 4.8 km/s), have sub Damped Lyman alpha 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.