1998A&A...339..745K

The large scale distributions of gas, magnetic field and cosmic rays in the galactic halo are investigated. Our model is based on the analysis of all-sky surveys of HI gas (Leiden/Dwingeloo survey), soft X-ray radiation (ROSATall-sky survey), high energy γ-ray emission (EGRET>100MeV), and radio-continuum emission (408MHz survey). We found a stable hydrostatic equilibrium configuration of the Galaxy which, on large scales, is consistent with the observations. Instabilities due to local pressure or temperature fluctuations can evolve only beyond a scale height of 4kpc. We have to distinguish 3 domains, with different physical properties and scale heights: 1) The gaseous halo has an exponential scale height h_z≃4.4kpc. Its radial distribution is characterised by a galactocentric scale length A₁≃15kpc. On large scales all components of the halo - gas, magnetic fields and cosmic rays - are in pressure equilibrium. The global magnetic field is regularly ordered and oriented parallel to the galactic plane. 2) The disk has a vertical scale height of about 0.4kpc. Characteristic for this region is the high gas pressure. The associated magnetic field is irregularly ordered and its equivalent pressure is only ≃1/3 of the gas pressure. The cosmic rays are decoupled from gas and magnetic fields. 3) The diffuse ionised gas layer with a vertical scale height of about 0.95kpc and a radial scale length of A₁≃15kpc acts as a disk-halo interface. The magnetic field in this region has properties similar to that in the disk. However, here the cosmic rays are coupled to the magnetic fields in contrast to the situation within the galactic disk. The gas pressure in this transition region is essential for the stability of the galactic halo system. Applying the model we can derive some major properties of the Milky Way: Assuming that the distribution of the gas in the halo traces the dark matter, we derive for a flat rotation curve a total mass of M=2.8x10¹¹M_☉. The mass of the galactic halo is M_halo≃2.1x10¹¹M_☉. We find that turbulent motions in the gaseous halo can be described by the Kolmogoroff relation. The smallest clouds, which are compatible with such a turbulent flow, are at temperatures close to 3K. They have linear sizes of ∼20au and masses of ∼2x10^–3M_☉. A significant fraction of the galactic dark matter may be in this form.