CO-dark molecular gas traced by HCO$^+$ in the diffuse interstellar medium
Year: 2026
Type: preprint
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 ($f_{\mathrm{CNM}} = 0.38^{+0.28}_{-0.27}$) and a lower fraction of hydrogen in H$_2$ ($f_{\mathrm{mol}}=0.09^{+0.06}_{-0.03}$) 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 $\lesssim10^{-6}$-$10^{-5}$ 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 $N(\mathrm{H_2})\lesssim\mathrm{few}\times10^{19}$ $\mathrm{cm^{-2}}$ - a result of both the low CO abundance and the low H$_2$ column - while HCO$^+$ absorption is readily detected for $N(\mathrm{H_2})\gtrsim\text{few}\times10^{18}$ $\mathrm{cm^{-2}}$. 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.
Source: arXiv (Cornell University)
Institutions: University of Wisconsin–Madison, National Radio Astronomy Observatory, Commonwealth Scientific and Industrial Research Organisation, Australia Telescope National Facility, Macquarie University, Korea Astronomy and Space Science Institute, Korea University of Science and Technology, Institut de Radioastronomie Millimétrique
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7163576397
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