Astrophys. J., 585, 801-822 (2003/March-2)
Fitting together the H I absorption and emission in the Southern Galactic Plane Survey.
DICKEY J.M., McCLURE-GRIFFITHS N.M., GAENSLER B.M. and GREEN A.J.
Abstract (from CDS):
In this paper we study 21 cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326°<l<333°) of the Southern Galactic Plane Survey. This survey combines the high resolution of the Australia Telescope Compact Array with the full brightness temperature information of the Parkes single-dish telescope. In particular, we focus on the abundance and temperature of the cool atomic clouds in the inner Galaxy. The resulting mean opacity of the H I, κ, 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 κ and Tcool.
ISM: Atoms - ISM: Clouds - ISM: Structure - Radio Lines: ISM
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