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New photoionization models of intergalactic cloudsNew photoionization models of optically thin low-density intergalactic gas at constant pressure, photoionized by QSOs, are presented. All ion stages of H, He, C, N, O, Si, and Fe, plus H2 are modeled, and the column density ratios of clouds at specified values of the ionization parameter of n sub gamma/n sub H and cloud metallicity are predicted. If Ly-alpha clouds are much cooler than the previously assumed value, 30,000 K, the ionization parameter must be very low, even with the cooling contribution of a trace component of molecules. If the clouds cool below 6000 K, their final equilibrium must be below 3000 K, owing to the lack of a stable phase between 6000 and 3000 K. If it is assumed that the clouds are being irradiated by an EUV power-law continuum typical of WSOs, with J0 = 10 exp -21 ergs/s sq cm Hz, typical cloud thicknesses along the line of sight that are much smaller than would be expected from shocks, thermal instabilities, or gravitational collapse are derived.
Document ID
19920035405
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Donahue, Megan (Joint Inst. for Lab. Astrophysics Boulder, CO, United States)
Shull, J. M. (Joint Institute for Laboratory Astrophysics; Colorado, University Boulder, United States)
Date Acquired
August 15, 2013
Publication Date
December 20, 1991
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 383
ISSN: 0004-637X
Subject Category
ASTROPHYSICS
Funding Number(s)
CONTRACT_GRANT: NAGW-766
Distribution Limits
Public
Copyright
Other