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

October 1995 • 1995ApJ...452..262S

Authors • Sodroski, T. J. • Odegard, N. • Dwek, E. • Hauser, M. G. • Franz, B. A. • Freedman, I. • Kelsall, T. • Wall, W. F. • Berriman, G. B. • Odenwald, S. F. • Bennett, C. • Reach, W. T. • Weiland, J. L.

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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