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 (H 2 ) in cold environments. Observations of CO emission are typically used to indirectly trace H 2 , but a significant fraction of H 2 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 H 2 in diffuse directions. In particular, a kinematically broad HCO + absorption signature traces extremely diffuse, CO-dark H 2 . 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 ( f CNM = 0.3 8 − 0.27 + 0.28 ) and a lower fraction of hydrogen in H 2 ( f mol = 0.0 9 − 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 H 2 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 × 10 19 cm −2 —a result of both the low CO abundance and the low H 2 column—while HCO + absorption is readily detected for N (H2) ≳ few × 10 18 cm −2 . These results demonstrate that even modest amounts of cold H I can bear H 2 , providing critical constraints on the H I -to-H 2 transition in the ISM.