HYDROSTATIC EQUILIBRIUM CONDITIONS IN THE GALACTIC HALO

Citation
Pmw. Kalberla et J. Kerp, HYDROSTATIC EQUILIBRIUM CONDITIONS IN THE GALACTIC HALO, Astronomy and astrophysics (Berlin), 339(3), 1998, pp. 745-758
Citations number
79
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Astronomy & Astrophysics
ISSN journal
0004-6361
Volume
339
Issue
3
Year of publication
1998
Pages
745 - 758
Database
ISI
SICI code
0004-6361(1998)339:3<745:HECITG>2.0.ZU;2-1
Abstract
The large scale distributions of gas, magnetic field and cosmic rays i n the galactic halo are investigated. Our model is based on the analys is of all-sky surveys of Hr gas (Leiden/Dwingeloo survey), soft X-ray radiation (ROSAT all-sky survey), high energy gamma-ray emission (EGRE T > 100 MeV), and radio-continuum emission (408 MHz 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 4 kpc. We have to distinguish 3 domains, with differe nt physical properties and scale heights. 1) The gaseous halo has an e xponential scale height h(z) similar or equal to 4.4 kpc. its radial d istribution is characterised by a galactocentric scale length A(1) sim ilar or equal to 15 kpc. On large scales all components of the halo - gas, magnetic fields and cosmic rays - are in pressure equilibrium. Th e global magnetic field is regularly ordered and oriented parallel to the galactic plane. 2) The disk has a vertical scale height of about 0 .4 kpc. Characteristic for this region is the high gas pressure. The a ssociated magnetic field is irregularly ordered and its equivalent pre ssure is only similar or equal to 1/3 of the gas pressure. The cosmic rays are decoupled from gas and magnetic fields. 3) The diffuse ionise d gas layer with a vertical scale height of about 0.95 kpc and a radia l scale length of Al - 15 kpc acts as a disk-halo interface. The magne tic field in this region has properties similar to that in the disk. H owever, here the cosmic rays ape coupled to the magnetic fields in con trast 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 ma ss of M = 2.8 10(11) M.. The mass of the galactic halo is M-halo simil ar or equal to 2.1 10(11) M.. We find that turbulent motions in the ga seous halo can be described by the Kolmogoroff relation. The smallest clouds, which are compatible with such a turbulent flow, are at temper atures close to 3 K. They have linear sizes of similar to 20 au and ma sses of similar to 2 10(-3) M.. A significant fraction of the galactic dark matter may be in this form.