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Photoionization in the halo of the GalaxyThe ionizing radiation field in the halo is calculated and found to be dominated in the 13.6-45 eV range by light from O-B stars that escapes the disk, by planetary nebulae at 45-54 eV, by quasars and the Galactic soft X-ray background at 54-2000 eV, and by the extragalactic X-ray background at higher energies. Photoionization models are calculated with this radiation field incident on halo clouds of constant density for a variety of densities, for normal and depleted abundances, and with variations of the incident spectrum. For species at least triply ionized, such as Si IV, C IV, N V, and O VI, the line ratios are determined by intervening gas with the greatest volume, which is not necessarily the greatest mass component. Column densities from doubly ionized species like Si III should be greater than from triply ionized species. The role of photoionized gas in cosmic ray-supported halos and Galactic fountains is discussed. Observational tests of photoionization models are suggested.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Bregman, Joel N.
(New York University NY; National Radio Astronomy Observatory, Charlottesville, VA, United States)
Harrington, J. Patrick
(Maryland, University College Park, United States)
Date Acquired
August 13, 2013
Publication Date
October 15, 1986
Publication Information
Publication: Astrophysical Journal, Part 1
Volume: 309
ISSN: 0004-637X
Subject Category
Accession Number
Funding Number(s)
Distribution Limits

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