The Ratio of H 2 Column Density to 12CO Intensity in the Vicinity of the Galactic Center

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Abstract

Observations from the COBE7 Diffuse Infrared Background Experiment (DIRBE) at wavelengths of 140 and 240 microns are combined with the Goddard-Columbia 12CO (J = 1 0) surveys to derive an estimate for X, the ratio of H₂ column density to 12CO intensity, within approximately 400 pc of the Galactic center. The H₂ column density is inferred from the infrared observations by assuming a proportionality between dust-to-gas mass ratio and gas metallicity It is found that the value of X in the Galactic center region is a factor of 3-10 lower than the corresponding ratio for molecular cloud complexes in the inner Galactic disk. Therefore, the use of the inner disk value of X to derive the mass of molecular hydrogen in the vicinity of the Galactic center and the 300 MeV-5 GeV gamma ray flux from that region will result in overestimates of both quantities. We attribute the so-called gamma-ray deficit from the Galactic center region to the erroneous use of a constant value of X throughout the Galaxy. Combining our results with several virial analyses of giant molecular cloud complexes in the Galactic disk, we find that the value of X increases by more than an order of magnitude from the Galactic center to a Galactocentric distance of 13 kpc. This implies that studies of the large-scale 12CO emission from our own Galaxy and external spiral galaxies, in which a constant ratio of H₂ column density to 12CO intensity was adopted, have significantly overestimated the relative amount of molecular hydrogen at small Galactocentric distances and significantly underestimated the relative amount of molecular hydrogen at large Galactocentric distances.

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