2018ApJS..235....1L

Hydroxyl (OH) is expected to be abundant in diffuse interstellar molecular gas because it forms along with H₂ under similar conditions and forms within a similar extinction range. We have analyzed absorption measurements of OH at 1665 MHz and 1667 MHz toward 44 extragalactic continuum sources, together with the J = 1-0 transitions of ¹²CO, ¹³CO, and C¹⁸O, and the J = 2-1 transition of ¹²CO. The excitation temperatures of OH were found to follow a modified lognormal distribution f(T_ex)∝1 / sqrt(2π)σ exp[ -[ln(T_ex)-ln(3.4K)]² / 2σ ²], the peak of which is close to the temperature of the Galactic emission background (CMB+synchrotron). In fact, 90% of the OH has excitation temperatures within 2 K of the Galactic background at the same location, providing a plausible explanation for the apparent difficulty of mapping this abundant molecule in emission. The opacities of OH were found to be small and to peak around 0.01. For gas at intermediate extinctions (A_V ∼ 0.05-2 mag), the detection rate of OH with a detection limit N(OH) ≃ 10¹² cm^–2 is approximately independent of A_V. We conclude that OH is abundant in the diffuse molecular gas and OH absorption is a good tracer of "dark molecular gas (DMG)." The measured fraction of DMG depends on the assumed detection threshold of the CO data set. The next generation of highly sensitive low-frequency radio telescopes, such as FAST and SKA, will make feasible the systematic inventory of diffuse molecular gas through decomposing, in velocity, the molecular (e.g., OH and CH) absorption profiles toward background continuum sources with numbers exceeding what is currently available by orders of magnitude.