Revealing the CO X-factor in dark molecular gas through sensitive ALMA absorption observations

Carbon-bearing molecules, particularly CO, have been widely used as tracers of molecular gas in the interstellar medium (ISM). In this work, we aim to study the properties of molecules in diffuse, cold environments, where CO tends to be underabundant and/or subthermally excited. We performed one of the most sensitive (down to τ^CO_rms ∼ 0.002 and τ^HCO+_rms ∼ 0.0008) submillimeter molecular absorption line observations toward 13 continuum sources with the ALMA. CO absorption was detected in diffuse ISM down to A_v < 0.32 mag and HCO⁺ was detected down to A_v < 0.2 mag , where atomic gas and dark molecular gas start to dominate. Multiple transitions measured in absorption toward 3C454.3 allow for a direct determination of excitation temperatures T_ex of 4.1 and 2.7 K, for CO and for HCO⁺, respectively, which are close to the cosmic microwave background and explain their being undercounted in emission surveys. A stronger linear correlation was found between N_HCO+ and N_H2 (Pearson correlation coefficient P ∼ 0.93) than that of N_CO and N_H2 (P ∼ 0.33), suggesting HCO⁺ is a better tracer of H₂ than CO in diffuse gas. The derived CO-to-H₂ conversion factor (the CO X-factor) of (14 ± 3) × 10²⁰ cm^-2 (K km s^-1) ^-1 is approximately six times larger than the average value found in the Milky Way.