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

Abstract

We modeled recent observations of UV absorption of HD and H2 in the Milky Way and toward damped/subdamped Lyα systems at z = 0.18 and z >1.7. N(HD)/N(H2) ratios reflect the separate self-shieldings of HD and H2 and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with 16 cm-3 ≲ n(H) ≲ 128 cm-3 if the cosmic-ray ionization rate per H nucleus ζH = 2 × 10-16 s-1, as inferred from H3+ and OH+. The dominant influence on N(HD)/N(H2) 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(H2). At z > 1.7, N(HD) is comparable to the Galaxy but with 10 times smaller N(H2) and somewhat smaller N(H2)/N(H i). Comparison of our Galaxy with theMagellanic Clouds shows that smaller H2/H is expected at subsolar metallicity, and we show by modeling that HD/H2 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/H2 at metallicity 0.04 ≲ Z ≲ 2 solar. In parallel, we trace dust extinction and self-shielding effects. The abrupt H2 transition to H2/H ≈ 1%-10% occurs mostly from self-shielding, although it is assisted by extinction for n(H) ≲ 16 cm-3. Interior H2 fractions are substantially increased by dust extinction below ≲ 32 cm-3. At smaller n(H), ζH , small increases in H2 triggered by dust extinction can trigger abrupt increases in N(HD)