The Galactic Dark Matter Halo: Is it H₂?

Summary

We discuss several recent observational results according to which a significant fraction of the Galactic dark matter halo may exist in the form of dense self-gravitating clumps of H₂.

We model the large scale mass distribution in the Milky Way and derive in a self-consistent way the associated gravitational potential. The basic constraint in our model is that the shape of the gaseous Galactic halo, observable from synchrotron radiation, γ-rays, soft X-ray background and from H I lines with high-velocity dispersion needs to be explained by the mass distribution.

The resulting mass model has a flat rotation curve. In the solar vicinity surface density, volume density and gravitational acceleration K_z are consistent with all known constraints. We find that the distributions of HI gas and dark matter are closely related to each other. Furthermore, the mass distribution implies a co-rotation of disk and halo for radii R > 5 kpc.

Our analysis supports strongly the hypothesis, that the gaseous halo of the Milky Way traces dark matter in the form of dense self-gravitating H₂ condensations as indicated from an analysis of the γ-ray background observed with EGRET (Shchekinov et al., these proceedings). In addition we find evidence for an extended non-baryonic dark matter halo, which co-exists with the baryonic component. We derive for the Milky Way a baryonic mass fraction of 12% in close agreement with cosmological predictions.