1420 MHz Continuum Absorption toward Extragalactic Sources in the Galactic Plane

Abstract

We present a 21 cm emission-absorption study toward extragalactic sources in the Canadian Galactic Plane Survey (CGPS). We have analyzed H I spectra toward 437 sources with Sν>=150 mJy, giving us a source density of 0.6 sources per square degree at arcminute resolution. We present the results of a first analysis of the H I temperatures, densities, and feature statistics. Particular emphasis is placed on five features with observed spin temperatures below 40 K. We find most spin temperatures in the range 40-300 K. A simple H I two-component model constrains the bulk of the cold component to temperatures (Tc) between 40 and 100 K. We find that Tc peaks in the Perseus arm region, and clearly drops off with Galactocentric radius R beyond that. The H I density follows this trend, ranging from a local value of 0.4 cm-3 to less than 0.1 cm-3 at R=20 kpc. We find that H I emission alone on average traces about 75% of the total H I column density, as compared to the total inferred by the emission and absorption. Comparing the neutral hydrogen absorption to CO emission, no correlation is found in general, but all strong CO emission is accompanied by a visible H I spectral feature. Finally, the number of spectral H I absorption features per kiloparsec drops off exponentially with increasing R.