Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...414...41m&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 414, no. 1, p. 41-56.
Astronomy and Astrophysics
Astrophysics
171
Dark Matter, Diffuse Radiation, Hydrogen Atoms, Neutral Gases, Radiation Distribution, Spiral Galaxies, Galactic Mass, H Alpha Line, Interstellar Gas, Interstellar Matter
Scientific paper
It is shown that the very sharp truncation of the neutral hydrogen distribution seen in NGC 3198 (and probably M33) is well modeled as the result of ionization of the atomic gas by the extragalactic radiation field. Below a critical column density of about a few times 10 exp 19/sq cm the gas is dominantly ionized and undetectable in the 21-cm line. It is inferred from the photoionization models that the total disk gas distribution in NGC 3198 is actually fairly axisymmetric. The critical column density for ionization is not a strong function of galaxy mass or mass distribution; thus, all galaxies should show a cutoff at approximately the same column density. Specific models of 3198 suggest that the extragalactic ionizing photon flux is 5000-10,000 photons/sq cm s.
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