Baryonic Dark Matter in the Milky Way

Abstract

We study the question whether some of the dark matter in the Milky Way may be baryonic. Considering a proposal by Pfenniger & Combes that dark matter might be associated with the Galactic disk, we study first the case whether such a model may be consistent at all. To model the mass distribution in the Milky Way we assume that dark matter may be contained in an isothermal sphere but also within the disk. We add bulge, bar, and two stellar disks, thick and thin. The gas is characterized by a molecular component, HI gas (cold and warm neutral medium), a diffuse ionized gas component, and a more extended halo component giving rise to the soft X-ray background, but also observable as faint HI lines. All disk components are assumed to be isothermal. A self-consistent solution of the Poisson-Boltzmann equation for mass distribution and gravitational potential in 3-D is discussed. We find a pronounced flaring for all of the isothermal disk components. The flaring derived from the model is in excellent agreement with observations. Oort limit and the local gravitational potential perpendicular to the plane agree with observations. In general, such a solution is consistent with all known observational constraints. The proposal that a disk-like dark matter component in the Milky Way exists in form of H2 clumpuscules is found to be very appealing. A “conspiracy” between gas and dark matter would be a natural consequence. Cloud-cloud collisions and evaporation of H2 clumpuscules would explain that we can observe an X-ray background and faint HI emission lines despite the fact that the cooling timescale is similar to the free fall timescale. The turbulent motion of the clumpuscules would be important for the stability of the Milky Way. At the same time turbulence would explain the scale height of the gaseous halo. Such a baryonic dark matter disk would be consistent with the expected baryon fraction. Similar, the angular momentum of the Milky Way disk would get easily explainable.