Fitting Together the H i Absorption and Emission in the Southern Galactic Plane Survey

In this paper we study 21 cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326deg, is measured as a function of Galactic radius; it increases going in from the solar circle, to a peak in the molecular ring of about 4 times its local value. This suggests that the cool phase is more abundant there, and colder, than it is locally. The distribution of cool-phase temperatures is derived in three different ways. The naive, spin temperature " technique overestimates the cloud temperatures as expected. Using two alternative approaches we get good agreement on a histogram of the cloud temperatures Tcool corrected for blending with warm-phase gas. The median temperature is about 65 K but there is a long tail reaching down to temperatures below 20 K. Clouds with temperatures below 40 K are common though not as common as warmer clouds (40-100 K). Using these results we discuss two related quantities the peak brightness temperature seen in emission surveys and the incidence of clouds seen in H i self-absorption. Both phenomena match what would be expected based on our measurements of and Tcool."