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Fitting Together the H i Absorption and Emission in the Southern Galactic Plane Survey
journal contributionposted on 2023-05-25, 23:04 authored by John DickeyJohn Dickey, McClure-Griffiths, NM, Gaensler, BM, Green, AJ
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."
Publication titleAstrophysical Journal
Rights statementCopyright 2003, The American Astronomical Society.