THE MULTIPHASE STRUCTURE OF THE GALACTIC HALO - HIGH-VELOCITY CLOUDS IN A HOT CORONA

Observations indicate that some high-velocity clouds (HVCs) have a two -phase structure consisting of cold cores and warm envelopes. We calcu late the thermal equilibrium gas temperature and investigate the therm al stability of neutral gas in the Galactic halo. Phase diagrams (ther mal pressure P vs. gas density n) are presented for gas at a range of heights z above the Galactic plane. Our method accounts for the photoe lectric heating from small grains and PAHs and includes a detailed tre atment of the ionization rates and heating due to the soft X-ray backg round and due to cosmic rays. We find that stable two-phase gas exists over a range of heights, but only within a narrow range of pressures at each height. Using a realistic Galactic gravitational potential and using halo parameters consistent with observed properties of the soft X-ray background, we show that a hot (T similar to 1-2 x 10(6) K) Gal actic corona can provide the necessary pressure for two-phase HVCs. We find that for an isothermal T = 10(6) K halo, the observed X-ray emis sion measure (EM(h) = 2.5 x 10(-3) pc cm(-6)) yields a thermal pressur e in the Galactic midplane of similar or equal to 2000-3000 K cm(-3), similar to observed pressures. In addition, we find that the electrons in the hot halo make a nonnegligible contribution to the dispersion m easure of pulsars far from the Galactic plane. A method is presented f or estimating HVC distances from X-ray shadowing measurements. We demo nstrate that the two-phase nature of HVCs can be used to constrain the distance to the clouds and their metallicity or origin. A primordial origin (extremely low metallicities) can be ruled out since two phases exist only for z < 2 kpc, contrary to most distance lower limits to H VCs. An extragalactic origin for some clouds, a result of gas stripped from the LMC, is supported by our calculated range of distances over which two-phase clouds exist in a hot corona, the observed dust and me tal content, and the high velocities observed for a few HVCs. We predi ct that no cold cores are expected to be found in a T = 10(6) K halo a t distances greater than similar to 20 kpc, which is consistent with t he lack of cold cores seen in the Magellanic Stream. However, the mass infall rate to the Galactic plane, and the general velocity distribut ion of halo clouds is consistent with an origin from gas injected into the halo by a Galactic fountain. For this case, two-phase models requ ire that the abundance of very small grains and PAHs in the fountain c louds must be similar to the standard abundances in the disk.