New constraints on CO-dark molecular gas properties in the diffuse ISM: Insights from 21cm and 3mm absorption observations

Observations of diffuse atomic and molecular gas in absorption. HI absorption (black) traces cold atomic gas. HCO+ (pink) and CO (blue) absorption are both thought to trace molecular hydrogen. Yet, clearly there is a significant amount of molecular gas traced by HCO+ with no corresponding CO. This is the “CO-dark” gas. The HCO+ absorption reveals that molecular gas is associated with nearly all the cold atomic gas in this direction.

Abstract:

The evolution of the multiphase interstellar medium (ISM) drives galaxy evolution. The transition from diffuse atomic gas to denser molecular clouds sets the initial conditions for star formation, so plays a critical role in this evolution. However, since molecular hydrogen (H2) is invisible in many diffuse environments, observational constraints on the atomic-to-molecular transition remain limited. CO emission is often used to trace H2, but in the diffuse ISM, a significant fraction of H2 is not associated with any CO emission (the “CO-dark” molecular gas). Thankfully, HCO+ absorption at 3mm is an excellent tracer of diffuse molecular gas — with a stable abundance and formation/survival at low column density, HCO+ is a reliable probe of the H2 column density in the diffuse ISM. Here we report observations of HI, HCO+, and CO in absorption toward five diffuse lines of sight. In all of these directions, we detect a kinematically broad, CO-dark signature in HCO+ absorption that is associated with nearly all the cold atomic gas in these directions. We constrain the CO abundances in these directions, which are consistent with chemical model predictions and account for the non-detection of CO. In fact, we show that CO emission or absorption are unlikely to be detected where N(H2) < 7E19 cm^-2, while HCO+ absorption is readily detected for N(H2) > ~fewE18 cm^-2, opening an important window on the HI-to-H2 transition. The diffuse, CO-dark gas associated with broad HCO+ absorption has systematically different properties, including a lower fraction of cold atomic gas and a lower molecular fraction, than CO-bright gas. These observations therefore offer insights on the HI-to-H2 transition that are inaccessible with CO observations.  I will briefly discuss future work aimed at uncovering the spatial structure of this diffuse, partially-molecular gas — key to understanding the physics of this critical stage of galaxy evolution.