In "The Dynamic Interstellar Medium: A Celebration of
the Canadian Galactic Plane Survey". Proceedings of a conference held at
the Naramata Centre, Naramata, British Columbia, Canada on 6-10 June
2010. Edited by R. Kothes, T. L. Landecker, and A. G. Willis. San
Francisco: Astronomical Society of the Pacific, 2010, p.179.
astro-ph/1008.0622
For a PDF version of the article, click here.
THE DISTRIBUTION OF WARM IONIZED MEDIUM IN GALAXIES
L.M. Haffner
Department of
Astronomy,
University of
Wisconsin-Madison,
475 North Charter Street, Madison, WI 53706
Abstract:
Ionized nebulae have been targets of interest since the introduction of
the telescope centuries ago. These isolated, "classical"
H ii regions gave us
some of the earliest insight into the copious feedback energy that stars
inject into the interstellar medium. Their unique spectra contain
information about the quality and quantity of the ionizing field as well
as the temperature, density, and metallicity of these discrete locations
in the Galaxy. With increasing sensitivity across many spectral domains,
we now know that ionized gas is not localized to massive star regions in
many star-forming galaxies. In particular, recent observational studies
allow a thorough comparison of the physical conditions and distribution
of the well-studied classical H II regions to the more widespread warm,
diffuse gas. By more realistically evolving a dynamic interstellar
medium, models are beginning to reproduce the observed emission measure
variations and provide a natural solution to the propagation of ionizing
flux from a predominantly neutral galactic disk to the distant halo.
Bright, ionized nebulae are one of the most obvious sites where feedback
from massive stars impacts the interstellar medium (ISM) in star-forming
galaxies. But the discovery and study of widespread ionized gas over a
wide range of temperatures (T ~ 104 - 106
K) throughout the last
century reveals that energy is deposited through other, large-scale
processes. One phase that has received considerable attention over the
last four decades is the pervasive, warm (~ 0.7 - 1.2 × 104
K) ionized medium (WIM). In most spiral galaxies with star formation
similar or more vigorous than the Milky Way, the WIM is a thick (~
few kpc) layer of gas with a high fraction
(
80%) of ionized
hydrogen.
Haffner et al. (2009a)
reviews much of the observational and
theoretical work done recently to define its properties and explore the
possibilities for powering the ionization and heating of this
component. Here, we review the history of early observations, issues
that arose from those first data, and early simulations that explored
solutions (Section 1 and
Section 2); more
recent extragalactic and Galactic observations made possible with modern
instrumentation (Section 3 and
Section 4); and
new simulations inspired by a growing appreciation for turbulence and
the complex density distribution of a real, dynamic ISM
(Section 5).
Table of Contents
