The gamma-ray deficit toward the Galactic Center.

The gamma-ray flux (E greater than 300 MeV) from the central few hundred parsecs of the Milky Way is shown to be nearly an order of magnitude smaller than the value expected from the H2 masses generally estimated to be present in the center and from the average gamma-ray emissivity measured for the disk. This result implies that in the Galactic center the gamma-ray emissivity is anomalously low, or that molecular hydrogen is nearly an order of magnitude less abundant than estimates made from CO observations. The former suggests that the density of cosmic-ray nuclei with energies of 1 GeV is anomalously small relative to the local value, or perhaps that these cosmic rays do not efficiently penetrate the molecular clouds in the central region of the Galaxy. The latter suggests that the gas-to-dust ratio is anomalously low and that the 3-sigma upper limit to the mass of the nuclear disk is 5.8 x 10 to the 7th solar mass. Circumstantial evidence favors a low H2/CO abundance as the source of the gamma-ray deficiency.