We investigate models for the photoionization of the widespread diffuse ionized gas in galaxies. In particular we address the long standing question of the penetration of Lyman continuum photons from sources close to the galactic midplane to large he
ights in the galactic halo. We find that recent hydrodynamical simulations of a supernova-driven interstellar medium have low density paths and voids that allow for ionizing photons from midplane OB stars to reach and ionize gas many kiloparsecs above the midplane. We find ionizing fluxes throughout our simulation grids are larger than predicted by one dimensional slab models, thus allowing for photoionization by O stars of low altitude neutral clouds in the Galaxy that are also detected in Halpha. In previous studies of such clouds the photoionization scenario had been rejected and the Halpha had been attributed to enhanced cosmic ray ionization or scattered light from midplane H II regions. We do find that the emission measure distributions in our simulations are wider than those derived from Halpha observations in the Milky Way. In addition, the horizontally averaged height dependence of the gas density in the hydrodynamical models is lower than inferred in the Galaxy. These discrepancies are likely due to the absence of magnetic fields in the hydrodynamic simulations and we discuss how magnetohydrodynamic effects may reconcile models and observations. Nevertheless, we anticipate that the inclusion of magnetic fields in the dynamical simulations will not alter our primary finding that midplane OB stars are capable of producing high altitude diffuse ionized gas in a realistic three-dimensional interstellar medium.
Feedback from massive stars is one of the least understood aspects of galaxy formation. We perform a suite of vertically stratified local interstellar medium (ISM) simulations in which supernova rates and vertical gas column densities are systematica
lly varied based on the Schmidt-Kennicutt law. Our simulations have a sufficiently high spatial resolution (1.95 pc) to follow the hydrodynamic interactions among multiple supernovae that structure the ISM. At a given supernova rate, we find that the mean mass-weighted sound speed and velocity dispersion decrease as the inverse square root of gas density, indicating that both thermal and turbulent pressures are nearly constant in the midplane, so the effective equation of state is isobaric. In contrast, across our four models having supernova rates that range from one to 512 times the Galactic supernova rate, the mass-weighted velocity dispersion remains in the range 4-6 km/s. Hence, gas averaged over ~100 pc regions follows an effective equation of state that is close to isothermal. Simulated H I emission lines have widths of 10-18 km/s, comparable to observed values. In our highest supernova rate model, superbubble blow-outs occur, and the turbulent pressure on large scales is >~4 times higher than the thermal pressure. We find a tight correlation between the thermal and turbulent pressures averaged over ~100 pc regions in the midplane of each model, as well as across the four ISM models. We construct a subgrid model for turbulent pressure based on analytic arguments and explicitly calibrate it against our stratified ISM simulations. The subgrid model provides a simple yet physically motivated way to include supernova feedback in cosmological simulations.
