Dense Gas Clouds and the Unidentified EGRET Sources

Abstract

Cold, dense gas clouds have been proposed as a major component of Galactic dark matter; such clouds can be revealed by the gamma-ray emission that arises from cosmic rays interacting with the gas. If this dark matter component is clustered, then highly luminous GeV sources result, preferentially at low-to-mid Galactic latitudes, where they lie within the cosmic-ray disk. The predicted emission for such clusters is steady, continuum emission with peak power emerging at several hundred MeV. These sources would not have obvious counterparts at other wavelengths and are thus of interest in connection with the Unidentified (UID) EGRET sources. Here we present a Monte Carlo simulation of the gamma-ray source population due to cold gas clouds, assuming a cold dark matter-like mass spectrum for the clustering. We find that ~280 EGRET sources are predicted by this model, with a median Galactic latitude of 12° for the population and a median angular size of 22. The latitude and size distributions are consistent with the UID EGRET source data, but the source counts are clearly overpredicted. On the basis of these results we propose that clusters of cold gas clouds comprise the majority population of the observed UID EGRET sources. Our interpretation implies that there should be microwave counterparts to most of the UID sources and will thus be strongly constrained by data from the present generation of microwave anisotropy experiments.