CO-dark molecular gas traced by HCO⁺ in the diffuse interstellar medium

A classic problem in the study of the interstellar medium (ISM) is the near-invisibility of molecular hydrogen (H₂) in cold environments. Observations of CO emission are typically used to indirectly trace H₂, but a significant fraction of H₂ in the diffuse ISM is not associated with any detectable CO emission (“CO-dark” molecular gas). Meanwhile, observations of HCO⁺ absorption trace nearly all of the H₂ in diffuse directions. In particular, a kinematically broad HCO⁺ absorption signature traces extremely diffuse, CO-dark H₂. We have used sensitive observations of HCO⁺, CO, and atomic hydrogen (H I) in absorption to constrain the properties of such diffuse molecular gas in five directions. The diffuse molecular gas revealed by broad HCO⁺ absorption has a lower fraction of cold H I (ƒ_CNM = 0.38_−0.27^+0.28) and a lower fraction of hydrogen in H₂ (ƒ_mol = 0.09_−0.03^+0.06) than gas traced by CO in the same directions. We detect almost no CO absorption from the gas traced by broad HCO⁺ absorption. We constrain the CO abundance relative to H₂ to be ≲10⁻⁶–10⁻⁵ for gas traced by both broad and narrow HCO⁺ absorption, consistent with chemical model predictions for the diffuse ISM. We further show that neither CO emission nor absorption is likely to be detected where N(H2) ≲ few × 10¹⁹ cm⁻²—a result of both the low CO abundance and the low H₂ column—while HCO⁺ absorption is readily detected for N(H2) ≳ few × 10¹⁸ cm⁻². These results demonstrate that even modest amounts of cold H I can bear H₂, providing critical constraints on the H I-to-H₂ transition in the ISM.