2015ApJ...799...66L

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/cm3 ≲ n(H) ≲ 128/cm3 if the cosmic-ray ionization rate per H nucleus ζ_H=2x10^–16/s, 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 the Magellanic 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/cm3. Interior H₂ fractions are substantially increased by dust extinction below ≲ 32/cm3. At smaller n(H), ζ_H, small increases in H₂ triggered by dust extinction can trigger abrupt increases in N(HD).