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Physical properties of neutral gas in M31 and the GalaxyThe present study analyzes, in parallel with published data for the Galaxy, neutral hydrogen (H I) absorption and deduced emission detected along seven lines of sight through the disk of M31. It is shown that the brightness temperature of H I emission is coupled to the opacity of the gas. The Galactic relationship shows asymptotic trends at both large and small opacities. A simple yet effective physical model which accounts for this behavior consists of only two independent components: a high-opacity cool component of fixed mean temperature, and a low-opacity warm component of fixed mean brightness. A lower mean gas pressure by a factor of about 2 is argued to be the most plausible mechanism for accounting for a higher cool-component H I temperature in M31. Deduced volume filling factors of the Galactic H I are about 1 and 15 percent, respectively, for the cool and warm components, while for M31 they are 8 and 30 percent. The large ratio of surface to volume filling factors for both cool and warm H I suggests that these components are distributed predominantly as large sheet- or shell-like structures.
Document ID
19920040715
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Braun, Robert
(Netherlands Foundation for Research in Astronomy, Dwingeloo; National Radio Astronomy Observatory, Socorro NM, United States)
Walterbos, Rene A. M.
(New Mexico State University Las Cruces, United States)
Date Acquired
August 15, 2013
Publication Date
February 10, 1992
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 386
ISSN: 0004-637X
Subject Category
Astrophysics
Accession Number
92A23339
Funding Number(s)
CONTRACT_GRANT: NAS5-26555
Distribution Limits
Public
Copyright
Other

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