CO-dark molecular gas traced by HCO${}^{+}$ in the diffuse interstellar medium

Abstract

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 H${}_2$ 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 (HI) 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 HI ($f_{\mathrm{CNM}}=0.38_{-0.27}^{+0.28}$) and a lower fraction of hydrogen in H${}_2$ ($f_{\mathrm{mol}}=0.09_{-0.03}^{+0.06}$) 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 H${}_2$ to be $\lesssim 10^{-6}$-$10^{-5}$ 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 $N({\mathrm{H}}_2)\lesssim \mathrm{few}\times 10^{19}$ $\mathrm{c}{\mathrm{m}}^{-2}$ - a result of both the low CO abundance and the low H${}_2$ column - while HCO${}^{+}$ absorption is readily detected for $N({\mathrm{H}}_2)\gtrsim \text{few}\times 10^{18}$ $\mathrm{c}{\mathrm{m}}^{-2}$. These results demonstrate that even modest amounts of cold HI can bear H${}_2$, providing critical constraints on the HI-to-H${}_2$ transition in the ISM.

Accepted for publication to ApJ; 20 pages, 8 figures