The Galactic Halo in Hydrostatic Equilibrium
Abstract
The large scale distribution of gas, magnetic field and cosmic rays in the Galaxy is explored. We analyze recent all-sky surveys of H I gas (Leiden/Dwingeloo survey, soft X-ray radiation (ROSAT) and high energy gamma-rays (EGRET > 100 MeV) in combination with the 408 MHz survey. >From these observations we derive a Galactic halo model consistent with hydrostatic equilibrium conditions. The large scale equilibrium configuration consists of two separate domains with different properties and scale sizes: 1) The gaseous halo consists of collisionally ionized gas at T=10^{6.2}K as well as H I and has an exponential scale height h_z ~4.4 kpc. The radial distribution is characterized by the galactocentric scale length A_1 ~15 kpc. All components of the halo - gas, magnetic field and cosmic rays - contribute equally to the pressure. The magnetic field in the halo is oriented parallel to the Galactic plane. 2) The disk extends up to a scale height of h_z ~1 kpc and is associated with irregular magnetic fields. Here the pressure of the magnetic field is only 1/3 of the gas pressure. The cosmic rays are only weakly coupled to the disk.
- Publication:
-
IAU Colloq. 166: The Local Bubble and Beyond
- Pub Date:
- 1998
- DOI:
- 10.1007/BFb0104768
- Bibcode:
- 1998LNP...506..475K