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Session 43 - The Diffuse ISM: Milky Way and Beyond.
Display session, Tuesday, June 11
We construct a multiphase model for the gaseous disk of the Milky Way galaxy with a goal of determining the physical conditions in the interstellar medium as a function of galactic radius. The models expand our previous work on the local interstellar medium to account for variations with galactic radius. The thermal equilibrium gas temperature of the neutral diffuse gas is calculated and phase diagrams (thermal pressure versus gas density) are presented for gas at galactic radii between 3 and 30 kpc. Our method accounts for the (far-ultraviolet) 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 explicitly calculate the galactic radial variation in thermal processes resulting from the radial variations of the space density of OB stars, (2) the metallicity and dust to gas ratio, (3) the (FUV) opacity, (4) the atomic and molecular gas surface density, and (5)the gas phase abundances of atomic coolants.
We find that the two neutral phases (CNM+WNM) can be in pressure equilibrium with a hot (T = 1-2\times 10^6 K) pervasive medium (HIM) for a wide range in galactic radii. The HIM extends high above the plane forming a halo. We take a model for the halo gas in which vertical isothermal columns are in hydrostatic equilibrium in the galactic gravitational potential. Using these simple assumptions, along with the constraint of the existence of two neutral (CNM+WNM) phases, we find that our halo model produces emission which is consistent with the observed soft X-ray background and provides an integrated luminosity from the galaxy which is comparable to that observed in other spirals. Our results will be important for an understanding of the distribution of molecular gas in the galaxy.
Program listing for Tuesday