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

Abstract

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 z < 2.3 are largely cool, while those seen at z > 2.8 are warm (though not devoid of H2). To interpret the observations of carbon and hydrogen we constructed detailed numerical models of H2 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 H2 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, H2 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.

Time-dependent H2 formation and protonation in diffuse clouds

“…We showed the approach to equilibrium under benign conditions for some recently-published models Liszt 2002Liszt , 2003, calculating the time-dependent H 2 and H 3 + formation in pc-size, otherwise largely static gas parcels meant to represent typical quiescent diffuse "clouds". We pointed out that self-shielding does not hasten the formation or accumulation of H 2 , although it may foster the production of more H 2 over longer times.…”

“…4) varying the assumed value of R G by factors of a few has appreciable effect upon the amount of H 2 which is produced but the equilibrium time scale changes little. As an aside we note that the gas-phase formation of H 2 via H − , which could account for some of the very low molecular fractions seen along lines of sight having low extinction (Liszt 2002), is included in the present calculations as well but without much effect.…”

“…That is, the initial model is equilibrated and self-consistent in terms of thermal and ionization equilibrium, but no molecule formation has occurred. Our models include the rudiments of molecular chemistry as discussed in several recent papers concerning questions of equilibrium CO, H 2 , HD and H 3 + formation in diffuse gas Liszt 2002Liszt , 2003. Incorporation of the chemistry should not normally upset the basic microphysical balance (ionization, thermodynamic, etc.)…”

“…Here we show the approach to equilibrium in the models discussed by , Liszt (2002) and Liszt (2003): we present calculations of time-dependent H 2 and H 3 + formation in small, mostly-static gas parcels meant to represent typical diffuse "clouds", like, for instance, the "standard" H I cloud of Spitzer (1978). As we discuss, the distinguishing characteristic of the chemistry in such clouds is that the H 2 formation process is unsaturated (n(H 2 ) < n(H)/2).…”

Cii* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two‐Phase Medium

“…We consider two models: one in which the line of sight to the QSO encounters gas in both CNM and WNM phases, and another in which it encounters only the WNM phase. The WNM model is considered because of recent arguments that DLAs consist only of WNM gas (Norman & Spaans 1997;Liszt 2002;Kanekar & Chengalur 2001). We find that the technique is insensitive to the masses and sizes of individual DLAs.…”