2024A&A...682A.161S

Context. Carbon monoxide (CO) is a poor tracer of H₂ in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules, unlike the situation at higher extinctions.
Aims. We present a novel, indirect method for constraining H₂ column densities (N_H2) without employing CO observations. We show that previously recognized nonlinearities in the relation between the extinction, A_V (H₂), derived from dust emission and the H I column density (N_H I_) are due to the presence of molecular gas.
Methods. We employed archival (N_H2) data, obtained from the UV spectra of stars, and calculated A_V(H₂) toward these sight lines using 3D extinction maps. The following relation fits the data: log N_H2 = 1.38742 (log A_V(H₂)⁾³ - 0.05359 (log A_V(H₂)⁾² + 0.25722 log A_V(H₂) + 20.67191. This relation is useful for constraining N_H2 in the diffuse ISM as it requires only N_H I_ and dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations of less than a factor of 3.5. We constructed a N_H2 map of our Galaxy and compared it to the CO integrated intensity (W_CO) distribution.
Results. We find that the average ratio (X_CO) between N_H2 and W_CO is approximately equal to 2 × 10²⁰ cm^–2 (K km s^–1 )^–1, consistent with previous estimates. However, we find that the X_CO factor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions with N_H2 ≳ 10²⁰ cm^–2, we estimate that the average H₂ fractional abundance, f_H2 = 2 N_H2/(2N_H2 + N_H I_), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines (A_V ≥ 1 mag), hence limiting f_H2 in these directions.
Conclusions. More than 50% of the lines of sight with N_H2 ≥ 10²⁰ cm^–2 are untraceable by CO with a J = 1-0 sensitivity limit W_CO = 1 K km s^–1.