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

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

Corresponding author: hliszt@nrao.edu

Received: 20 March 2002
Accepted: 29 April 2002

Abstract

Molecular hydrogen is quite underabundant in damped Lyman-α 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-α 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-α systems – in whole or part – is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman-α systems observed at lower redshift are largely cool, while those seen at 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-α 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 is expected.