Constraining the H2 column densities in the diffuse interstellar medium using dust extinction and Hi data

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 to constrain H2 column densities (NH2 ) without employing CO observations. We show that previously–recognized nonlinearities in the relation between the extinction, Av(H2), derived from dust emission and the Hi column density (NHI ), are due to the presence of molecular gas. Methods.We employ archival NH2 data, obtained from the UV spectra of stars, and calculate Av(H2) towards these sight lines using 3D extinction maps. The following relation fits the data: log NH2 = 1.38742log Av(H2)^3 − 0.05359log Av(H2)^2 + 0.25722log Av(H2) − 20.67191. This relation is useful for constraining NH2 in the diffuse ISM as it requires only NHI and dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations under a factor of 3.5. We construct a NH2 map of our Galaxy and compare it to the CO integrated intensity (WCO) distribution. Results. We find that the average ratio (XCO) between NH2 and WCO is approximately equal to 2 × 1020 cm−2 (K km s−1)−1, consistent with previous estimates. However, we find that the XCO factor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions with NH2 ≳ 1020 cm−2, we estimate that the average H2 fractional abundance, fH2= 2 NH2 / (2NH2 + NHI ), is 25%. Multiple (distinct) largely atomic clouds are likely found along high–extinction sightlines (Av ≥ 1 mag), hence limiting fH2 in these directions. Conclusions. More than 50% of the lines of sight with NH2 ≥ 1020 cm−2 are untraceable by CO with a J = 1–0 sensitivity limit of 1 K kms−1.