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

Molecular hydrogen is quite underabundant in damped Lyman-alpha 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-alpha 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-alpha systems - in whole or part - is actually cool; spectroscopy of neutral and ionized carbon, discussed here, shows that the damped Lyman-alpha systems observed at lower redshift z < 2.3 are largely cool, while those seen at z > 2.8 are warm (though not devoid of H-2). To interpret the observations of carbon and hydrogen we constructed detailed numerical models of H-2 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-alpha systems is enough to suppress H-2 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, H-2 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.