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Issue A&A Volume 389, Number 2, July II 2002 Page(s) 393 - 404 Section Extragalactic astronomy DOI 10.1051/0004-6361:20020646

A&A 389, 393-404 (2002)
DOI: 10.1051/0004-6361:20020646

Two-phase equilibrium and molecular hydrogen formation in damped Lyman-alpha systems

H. Liszt

National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

(Received 20 March 2002 / Accepted 29 April 2002 )

Abstract
Molecular hydrogen is quite underabundant in damped Lyman- systems at high redshift, when compared to the interstellar medium near the Sun. This has been interpreted as implying that the gas in damped Lyman- systems is warm like the nearby neutral intercloud medium, rather than cool, as in the clouds which give rise to most H I absorption in the Milky Way. Other lines of evidence suggest that the gas in damped Lyman- systems - in whole or part - is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman- systems observed at lower redshift z < 2.3 are largely cool, while those seen at z > 2.8 are warm (though not devoid of ). To interpret the observations of carbon and hydrogen we constructed detailed numerical models of formation under the conditions of two-phase thermal equilibrium, like those which account for conditions near the Sun, but with varying metallicity, dust-gas ratio, etc. We find that the low metallicity of damped Lyman- systems is enough to suppress formation by many orders of magnitude even in cool diffuse clouds, as long as the ambient optical/uv radiation field is not too small. For very low metallicity and under the most diffuse conditions, formation will be dominated by slow gas-phase processes not involving grains, and a minimum molecular fraction in the range 10^-8-10^-7 is expected.

Key words: quasars: absorption lines -- ISM: molecules

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