Multiphase Structure of the Galactic Halo
Session 114 -- Multi-Phase Interstellar Medium
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

## [114.03] Multiphase Structure of the Galactic Halo


We calculate the thermal equilibrium gas temperature of high velocity clouds (HVCs) in the Galactic Halo. 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. Phase diagrams (thermal pressure $P$ versus gas density $n$) are presented for gas with a range of dust/gas ratios (D/G) and a range of metallicities (Z). Variations in D/G affect mainly the photoelectric heating rate, while variations in Z affect both the photoelectric heating and gas cooling. Curves are shown for D/G = 1 (local value) to D/G\al 0.005 and for Z=1 (local value) to Z$= 0.005$. We find that a two phase medium (CNM + WNM) can be in pressure equilibrium with a hot ($T\sim 1-2\times 10^{6}$ K) halo within a range of permitted pressures, $P^{\rm min}$ to $P^{\rm max}$. We take halo parameters consistent with observed properties of the soft X-ray background. In general, both $P^{\rm min}$ and $P^{\rm max}$ decrease with lower D/G due to a drop in photoelectric heating from grains, while $P^{\rm min}$ and $P^{\rm max}$ increase with lower Z due to a drop in gas coolants.

We demonstrate that successful two phase models can be constructed with pressure in the range $10^{3} \al P/k \al 10^4$ K cm$^{-3}$ consistent with the thermal pressure in the Galactic disk. In addition, using the observed relation between CNM density and distance in HVCs, ($n=75/fD_{\rm kpc}$ cm$^{-3}$; Wakker \& Schwarz 1991, AA, 250, 484) we show that our pressure curves constrain the allowed range of HVC heights to be between 0.3 - 16 kpc.