2002/07/31 by M. G. Wolfire, C. F. McKee, D. Hollenbach +1 · 1 citation
Physics and Astronomy · #astro-ph
paper · pdf · doi:10.1086/368016
published as Astrophys.J. 587 (2003) 278-311 · 78 pages plus 19 figures, Accepted for publication in ApJ (Abridged abstract)
arxiv created 2002/12/20 · arxiv updated 2009/12/01
Much of the interstellar medium in disk galaxies is in the form of neutral atomic hydrogen, H I. This gas can be in thermal equilibrium at relatively low temperatures, T < 300 K (the cold neutral medium, or CNM) or at temperatures somewhat less than 104 K (the warm neutral medium, or WNM). These two phases can coexist over a narrow range of pressures, Pmin < P < Pmax. We determine Pmin and Pmax in the plane of the Galaxy as a function of Galactocentric radius R using recent determinations of the gas heating rate and the gas phase abundances of interstellar gas. We provide an analytic approximation for Pmin as a function of metallicity, far-ultraviolet radiation field, and the ionization rate of atomic hydrogen. Over most of the disk of the Galaxy, the H I must be in two phases: the weight of the H I in the gravitational potential of the Galaxy is large enough to generate thermal pressures exceeding Pmin, so that turbulent pressure fluctuations can produce cold gas that is thermally stable; and the mean density of the H I is too low for the gas to be all CNM. Our models predict the presence of CNM gas to R = 16-18 kpc, somewhat farther than previous estimates. We also examine the potential impact of turbulent heating on our results and provide expressions for the heating rate as a function of Galactic radius.