Constraining the H2 column densities in the diffuse interstellarmedium using dust extinction and HI data

Abstract

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