HD/H2 as a probe of the roles of gas, dust, light, metallicity and cosmic rays in promoting the growth of molecular hydrogen in the diffuse interstellar medium

TL;DR

This study models UV absorption observations of HD and H2 in the Milky Way and distant systems, revealing how cosmic rays, dust, and metallicity influence molecular hydrogen formation and HD/H2 ratios in the diffuse interstellar medium.

Contribution

It provides a detailed model linking HD/H2 ratios to cosmic-ray ionization, metallicity, and dust effects, explaining observations across different galactic environments.

Findings

Cosmic-ray ionization rate strongly influences HD/H2 ratios.

Dust extinction and self-shielding significantly affect molecular hydrogen transition.

HD/H2 ratios vary with metallicity and density, matching observations from local to high-redshift systems.

Abstract

We modelled recent observations of UV absorption of HD and \HH\ in the Milky Way and toward damped/sub-damped Lyman alpha systems at z=0.18 and z $>$ 1.7. N(HD)/N(\HH) ratios reflect the separate self-shieldings of HD and \HH\ and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with $ 16 \pccc \la$ n(H) $\la 128 \pccc $ if the cosmic-ray ionization rate per H-nucleus \zetaH $= 2\times 10^{-16}\ps$ as inferred from \H3\p\ and OH\p. The dominant influence on N(HD)/N(\HH) is the cosmic-ray ionization rate with a much weaker downward dependence on n(H) at Solar metallicity, but dust-extinction can drive N(HD) higher as with N(\HH). At z $>$ 1.7, N(HD) is comparable to the Galaxy but with 10x smaller N(\HH) and somewhat smaller N(\HH)/N(H I). Comparison of our Galaxy and the Magellanic Clouds shows that smaller \HH/H is expected at sub-Solar metallicity and we show by modelling that HD/\HH\ increases with density at low metallicity, opposite to the Milky Way. Observations of HD would be explained with higher n(H) at low metallicity but high-z systems have high HD/\HH\ at metallicity 0.04 $\la$ Z $\la$ 2 Solar. In parallel we trace dust-extinction and self-shielding effects. The abrupt \HH\ transition to \HH/H $\approx$ 1-10% occurs mostly from self-shielding although it is assisted by extinction for n(H) $\la 16 \pccc$. Interior \HH\ fractions are substantially increased by dust extinction below $\la 32\pccc$. At smaller n(H), \zetaH, small increases in \HH\ triggered by dust extinction can trigger abrupt increases in N(HD).