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

Abstract

Observations from the COBE<SUP>7</SUP> Diffuse Infrared Background Experiment (DIRBE) at wavelengths of 140 and 240 microns are combined with the Goddard-Columbia <SUP>12</SUP>CO (J = 1 0) surveys to derive an estimate for X, the ratio of H<SUB>2</SUB> column density to <SUP>12</SUP>CO intensity, within approximately 400 pc of the Galactic center. The H<SUB>2</SUB> 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. <P />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 <SUP>12</SUP>CO emission from our own Galaxy and external spiral galaxies, in which a constant ratio of H<SUB>2</SUB> column density to <SUP>12</SUP>CO 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.