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Photoionization of Galactic Halo Gas by Old Supernova Remnants

We present new calculations on the contribution from cooling hot gas to the photoionization of warm ionized gas in the Galaxy. We show that hot gas in cooling supernova remnants (SNRs) is an important source of photoionization, particularly for gas in the halo. We find that in many regions at high latitude this source is adequate to account for the observed ionization, so there is no need to find ways to transport stellar photons from the disk. The flux from cooling SNRs sets a floor on the ionization along any line of sight. Our model flux is also shown to be consistent with the diffuse soft X-ray background and with soft X-ray observations of external galaxies. We consider the ionization of the clouds observed toward the halo star HD 93521, for which there are no O stars close to the line of sight. Along this line of sight, two groups of clouds (densities ~0.3-1 cm-3 and temperatures ~7000 K) are observed at ~0 km s-1 ("slow") and ~-50 km s-1 ("fast"). We show that the observed ionization can be explained successfully by our model EUV/soft X-ray flux from cooling hot gas. In particular, we can match the Hα intensity, the S++/S+ ratio, and the C+* column. Our models show that it is possible to account for the observed ionization without invoking exotic ionization mechanisms such as decaying neutrinos (Sciama). Our value for X-ray opacity along this line of sight is somewhat larger than the average for the halo found by Arabadjis & Bregman, but we do not regard the difference as significant. From observations of the ratios of columns of C+* and either S+ or H0, we are able to estimate the thermal pressure in the clouds. The slow clouds require high (~104 cm-3 K) thermal pressures to match the N/N ratio. Additional heating sources are required for the slow clouds to maintain their ~7000 K temperatures at these pressures, as found by Reynolds, Haffner, & Tufte. We also estimate the fraction of the line of sight that is occupied by warm ionized and neutral gas and find it to be small, only 6%-9%.