A survey of H I absorption at low latitudes

Abstract

The VLA in phased-array mode was used to measure 21 cm absorption spectra toward 88 compact sources with /b/ less than 15 deg. Absorption spectra with velocity resolution 1.55 km/s and useful bandwidth greater than or equal to 260 km/s are presented. Corresponding emission spectra taken with the Green Bank 300 foot (91 m) telescope are also shown. In emission several points separated by 10-arcmin have been observed to indicate the level of variation to be expected on the scale of the 300 foot telescope beam. The most impressive contrasts between corresponding emission and absorption spectra are that absorption lines are narrower than the corresponding emission lines and that absorption spectra show gaps between the features where the optical depth drops to zero.