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 calculate the thermal equilibrium gas temperature and investigate the thermal stability of neutral gas in the Galactic halo. Phase diagrams (thermal 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 photoelectric heating from small grains and PAHs and includes a detailed treatment of the ionization rates and heating due to the soft X-ray background 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 ∼ 1-2 × 106 K) Galactic corona can provide the necessary pressure for two-phase HVCs. We find that for an isothermal T = 106 K halo, the observed X-ray emission measure (EMh = 2.5 × 10-3 pc cm-6) yields a thermal pressure in the Galactic midplane of ≌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 measure of pulsars far from the Galactic plane. A method is presented for estimating HVC distances from X-ray shadowing measurements. We demonstrate 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 6 K halo at distances greater than ∼20 kpc, which is consistent with the lack of cold cores seen in the Magellanic Stream. However, the mass infall rate to the Galactic plane, and the general velocity distribution of halo clouds is consistent with an origin from gas injected into the halo by a Galactic fountain. For this case, two-phase models require that the abundance of very small grains and PAHs in the fountain clouds must be similar to the standard abundances in the disk.