2023ApJ...943..172L

There is a robust polyatomic chemistry in diffuse, partially molecular interstellar gas that is readily accessible in absorption at radio/millimeter/submillimeter wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H₂. Here we consider the use of proxies for hydrogen column densities N(H₂) and N(H) = N(H I)+2N(H₂) based on measurements of HCO⁺ absorption and CO emission and absorption, and we compare these with results obtained by others when observing H I, H₂ and CO toward stars and active galactic nuclei. We consider the use of HCO⁺ as a proxy for H₂ and show that the assumption of a relative abundance N(H₂) = N(HCO⁺)/3 × 10^–9 gives the same view of the atomic-molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected when N(HCO⁺) ≳ 6 × 10¹¹ cm^–2 or N(H₂) ≳ 2 × 10²⁰ cm^–2. Wide variations in the integrated CO J = 1 - 0 brightness W_CO and N(CO)/N(H₂) imply equivalent variations in the CO-H₂ conversion factor even while the ensemble mean is near the usual Galactic values. Gas/reddening ratios found in absorption toward stars, N(H)/E(B - V) = 6.2 × 10²¹ H cm^–2 (mag)^–1 overall or 6.8 × 10²¹ H cm^–2 (mag)^–1 for sightlines at E(B - V) ≤ 0.08 mag lacking H₂ are well below the Galactic mean measured at low reddening and high Galactic latitude, 8.3 × 10²¹ H cm^–2 (mag)^–1.