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

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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 (gtapprox 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

DISCOVERY & EARLY EXPLORATION

EARLY MODELS

AN EXTRAGALACTIC PERSPECTIVE

GALACTIC Halpha IMAGING SURVEYS AND WHAM

NEW SIMULATIONS

SUMMARY

REFERENCES

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