Studies of the Interstellar Medium in the Inner

Galaxy at 21 Centimeters

By Robert Walter Garwood

Under the supervision of Dr. John Dickey


The results from a VLA snapshot survey of the galactic plane in the longitude range 0° < l < 90° at 1.5 GHz are presented. The positions and peak and total flux densities of 471 sources were measured. The survey is sensitive to sources smaller that about 40". The resolution is 4". An excess number of sources above that expected from extragalactic source counts is seen fro l < 40° at all flux density intervals. This excess is primarily due to galactic sources associated with radio recombination lines sources. There must only be a small number of galactic objects present in the survey which remain unidentified as such.

Twenty-one of the brightest compact sources found in the continuum survey are used as background sources for a 21cm HI absorption survey. The 21cm velocity-averaged absorption coefficient is calculated as a function of galactocentric radius. This quantity decreases toward the galactic center from a solar neighborhood value of 5 km/s/kpc, in agreement with high latitude absorption surveys, to roughly one-fourth of this value within 2 kpc from the galactic center. This decrease is shown primarily due to an increase in the mean line-of-sight distance between absorbing atomic clouds. Cool atomic gas is considerably less abundant in the inner galaxy. This is a surprising result. The absorption spectra are compared with existing 12 CO emission spectra. After correcting for velocity gradients effects, 85% of the velocities which show some HI absorption also show some CO emission.

The results of a search for infrared counterparts to the compact sources found in the VLA survey are presented. Nine red sources likely to be associated with a radio source were found. Most of these seem moderately hot, distant main sequence stars and their associated HII regions. Three sources have radio luminosities considerably brighter than expected fro this explanation. Some exotic possibilities are considered.