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Date and Time of the Query: 2019-06-15 T16:26:39 PDT
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For refcode 1993ApJ...419..515R:
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1993ApJ...419..515R NEUTRAL HYDROGEN IN GALAXIES AT THE PRESENT EPOCH SANDHYA RAO AND FRANK BRIGGS Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260 Received 1993 March 8; accepted 1993 July 2 ABSTRACT The evolution of the neutral hydrogen content of galaxies as a function of time is an important constraint on processes in galactic evolution. We present a comprehensive, statistical description of the H I content and distribution within galaxies at the present epoch and compare these statistics with the properties of H I associated with "damped Lyman-{alpha}" absorption systems at high redshift that are observed in the spectra of QSOs. {OMEGA}_HI_ (z = 0), the H I mass density at the present epoch relative to the present critical mass density, is found to be (2.5 +/- 0.6) x 10^-4^h^_75_^-1^ consistent with the decreasing trend of the H I content with time deduced from QSO absorption line statistics for redshifts from about 4 to 0.5 (Lanzetta 1993). Spiral galaxies contain an overwhelming 89% of this neutral hydrogen mass. The rest is contained in irregulars, S0s, and ellipticals. Spirals also offer the largest cross section to line-of-sight absorption of light from QSOs. By considering nearby spirals as potential absorbers, the interception probability as a function of the H I column density, N(H I), is derived for comparison with the cross sections inferred from observations of damped Lyman-{alpha} systems. The comparison shows that the damped Ly{alpha} lines are created by absorbers that subtend larger cross sections than present-day spirals by a factor of 5 implying that galaxies were either larger or more numerous at z 2.5. Subject headings: galaxies: evolution - galaxies: ISM - galaxies: luminosity junction, mass function - radio lines: galaxies
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