H2 in the diffuse ISM

We constructed a NH_2_ map of our Galaxy and compared it to the CO integrated intensity (W_CO_) distribution. We find that the average ratio (X_CO_) between NH_2_ and W_CO_ is approximately equal to 2x10^20cm^-2^(K.km/s)^-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 NH_2_>=10^20^cm^-2^, we estimate that the average H_2_ fractional abundance is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines (AV>=1mag), hence limiting fH_2_ in these directions. More than 50% of the lines of sight with NH_2_>=10^20cm^-2^ are untraceable by CO with a J=1-0 sensitivity limit WCO=1K.km/s.