| Type |
Journal Article |
| Names |
Kenneth Wood, Alex S. Hill, M. Ryan Joung, Mordecai-Mark Mac Low, Robert A. Benjamin, L. Matthew Haffner, R. J. Reynolds, G. J. Madsen |
| Publication |
The Astrophysical Journal |
| Volume |
721 |
| Issue |
2 |
| Pages |
1397-1403 |
| Date |
October 1, 2010 |
| URL |
http://adsabs.harvard.edu/abs/2010ApJ...721.1397W |
| Library Catalog |
NASA ADS |
| Abstract |
We investigate models for the photoionization of the widespread diffuse
ionized gas (DIG) 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 heights in the galactic
halo. We find that recent hydrodynamical simulations of a
supernova-driven interstellar medium (ISM) 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 that
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 Hα. In previous studies of such clouds,
the photoionization scenario had been rejected and the Hα 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
Hα 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 DIG in a realistic three-dimensional ISM. |
| Tags |
ISM: structure |