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


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Dickey, JM, McClure-Griffiths, NM, Gaensler, BM and Green, AJ 2003 , 'Fitting Together the H i Absorption and Emission in the Southern Galactic Plane Survey' , Astrophysical Journal, vol. 585, no. 1 , pp. 801-822 , doi: 10.1086/346081.

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In this paper we study 21 cm absorption spectra and the corresponding emission spectra toward bright continuum sources in the test region (326deg<l<333deg) 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, <kappa>, 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 <kappa> and Tcool.

Item Type: Article
Authors/Creators:Dickey, JM and McClure-Griffiths, NM and Gaensler, BM and Green, AJ
Keywords: atoms: clouds: structure: radio lines
Journal or Publication Title: Astrophysical Journal
DOI / ID Number: 10.1086/346081
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Copyright 2003, The American Astronomical Society.

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