Highly Ionized Gas in Galactic Halos

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Session 26 -- Interstellar Medium and Radiative Shocks
Oral presentation, Monday, 9, 1995, 10:00am - 11:30am

[26.01D] Highly Ionized Gas in Galactic Halos

Robert A. Benjamin (University of Minnesota)

We have calculated the time-dependent, nonequilibrium thermal and ionization history of gas cooling radiatively from $10^{6} K$ in a one-dimensional, planar, steady-state flow model of the galactic fountain, including the effects of radiative transfer. We show that the inclusion of the effects of photoionizing radiation emitted by the cooling gas itself or "self-ionization" is sufficient to cause the flow to match the observed galactic halo column densities of C IV, Si IV, and N V and UV emission from C IV and O III, for cooling region sizes, i.e. $D_{0} \, ^{>}_{\sim} \,15\,pc$. For an initial flow velocity $v_{0} \sim100\,km/s$, comparable to the sound speed of a $10^{6} K$ gas, the initial density is found to be $n_{H,0} \sim\, 2\,\times\,10^{-2} \,cm^{-3}$, in reasonable agreement with other observational estimates of ionized halo gas, and $D_{0} \sim\,40\,pc$. We also compare predicted $H \alpha$ fluxes, total ionizing flux, free-free radio emission, and broadband X-ray fluxes with observed values. In addition we apply the same fountain flow model to Lyman limit quasar metal absorption lines at higher redshift, showing that such models are also capable of matching the observational data. A unique prediction of our models is the presence of appreciable and potentially detectable column densities of Ne VIII.

We demonstrate the possibility that a transverse magnetic field in a radiative shock provides an alternative explanation for the occurrence of the isochoric cooling phase required in our steady state cooling flow model. Such a field can lead naturally to a constant downstream gas density once magnetic pressure increases enough to dominate the total pressure of the gas. We find that preshock parameters of $n_{H,1}=0.005\,cm^{-3}$ and $B_{1}=0.5 \mu G$ with a shock velocity of $v_{s}=300 km/s$ matches the observed column density ratios. We have calculated theoretical UV absorption line profiles and show how these profiles may be used to distinguish between different models for the origin of highly ionized gas.

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