Published in Rev. Mod. Phys. 81:969, 2009
astro-ph/0901.0941

For a PDF version of the article, click here.

THE WARM IONIZED MEDIUM IN SPIRAL GALAXIES

L. M. Haffner
Department of Astronomy, University of Wisconsin-Madison, Madison, Wisconsin USA, 53706

R.-J. Dettmar
Astronomisches Institut, Ruhr-University Bochum, Germany

J. E. Beckman
Instituto de Astrofísica de Canarias, Tenerife, Spain and Consejo Superior de Investigaciones Científicas, Madrid, Spain

K. Wood
University of St. Andrews, St. Andrews, Scotland

J. D. Slavin
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA

C. Giammanco
Instituto de Astrofísica de Canarias, Tenerife, Spain

G. J. Madsen
School of Physics, The University of Sydney, Sydney, NSW 2006, Australia

A. Zurita
Departamento de Física Teórica y del Cosmos, Universidade de Granada, Granada, Spain

R. J. Reynolds
Department of Astronomy, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA


Abstract. This article reviews observations and models of the diffuse ionized gas that permeates the disk and halo of our Galaxy and others. It was inspired by a series of invited talks presented during an afternoon scientific session of the 65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory in August 2004. This review is in recognition of Carl's long standing interest in and advocacy for studies of the ionized as well as the neutral components of the interstellar medium.


Table of Contents

INTRODUCTION

VIEW FROM THE INSIDE: DIVING INTO THE PHYSICS OF THE WARM IONIZED MEDIUM IN OUR GALAXY
Basic characteristics of the WIM and diagnostic tools
Ionization state
Temperature
[N II], [S II] and [O II] with respect to Halpha
[N II] l5755 / [N II] lambda6583
Line widths
Warm ionized and neutral gas
The role of superbubbles

VIEWS FROM THE OUTSIDE: DIFFUSE IONIZED GAS AND STAR FORMATION RATES IN EDGE-ON GALAXIES
Diffuse ionized gas in the halo and star formation in the disk
The disk-halo connection and hot gas

VIEWS FROM THE OUTSIDE: THE SOURCE OF THE DIFFUSE IONIZED GAS IN FACE-ON GALAXIES
Radiation from O stars and the surface brightness of the DIG
An escape model for Lyman continuum propagation
Line ratio studies

MODELING THE WIM/DIG: EFFECTS OF RADIATION TRANSFER THROUGH A CLUMPY INTERSTELLAR MEDIUM
What exactly is an H ii region?
Modeling the ionization structure of the DIG
Escape of ionizing radiation through superbubbles
Two- and three-dimensional ionization structure of the DIG
Modeling the emission line spectrum of diffuse ionized gas
One-dimensional models
Two- and three-dimensional models
Interfaces and three-dimentional H ii regions
Leaky H ii regions and the He+/H+ problem

IONIZING RADIATION FROM HOT GAS-COOL GAS INTERFACES
Types of interfaces
A test case

SOME QUESTIONS FOR FUTURE STUDY
What is the source of the elevated temeratures?
What is the spatial distribution of the gas?
How much ionizing radiation escapes the galaxy?
Do hot, pre-white dwarf stars play a role?
Is missing atomic data important?
What insights will new global models provide?

A LIST OF ACRONYMS AND TERMS

REFERENCES

Next