The warm ionized medium in spiral galaxies

📝 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.

💡 Analysis

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.

📄 Content

The warm ionized medium in spiral galaxies L.M. Haffner Department of Astronomy, University of Wisconsin–Madison, USA R.-J. Dettmar Astronomisches Institut, Ruhr-University Bochum, Germany J.E. Beckman Instituto de Astrof´ısica de Canarias, Tenerife, Spain and Consejo Superior de Investigaciones Cient´ıficas, Spain K. Wood University of St Andrews, Scotland J.D. Slavin Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA C. Giammanco Instituto de Astrof´ısica de Canarias, Tenerife, Spain G.J. Madsen School of Physics, The University of Sydney, NSW 2006, Australia A. Zurita Dept. de F´ısica Te´orica y del Cosmos, U. de Granada, Granada, Spain R.J. Reynolds Department of Astronomy, University of Wisconsin–Madison, USA 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. Contents I. Introduction 2 II. View from the inside: diving into the physics of the warm ionized medium in our galaxy 2 A. Basic characteristics of the WIM and diagnostic tools 2 B. Ionization state 4 C. Temperature 5

1. [N ii], [S ii] and [O ii] with respect to Hα 5
2. [N ii] λ5755/[N ii] λ6583 6
3. Line widths 6 D. Warm ionized and neutral gas 7 E. The role of superbubbles 9 III. Views from the outside: diffuse ionized gas and star formation rates in edge-on galaxies 9 A. Diffuse ionized gas in the halo and star formation in the disk 10 B. The disk-halo connection and hot gas 10 IV. Views from the outside: The source of the diffuse ionized gas in face-on galaxies 11 A. Radiation from O stars and the surface brightness of the DIG 11 B. An escape model for Lyman continuum propagation 12 C. Line ratio studies 14 V. Modeling the WIM/DIG: Effects of radiation transfer through a clumpy interstellar medium 15 A. What exactly is an H ii region? 15 B. Modeling the ionization structure of the DIG 17
4. Escape of ionizing radiation through superbubbles 17
5. Two- and three-dimensional ionization structure of the DIG 17 C. Modeling the emission line spectrum of diffuse ionized gas 18
6. One-dimensional models 19
7. Two- and three-dimensional models 19
8. Interfaces and three-dimentional H ii regions 19
9. Leaky H ii regions and the He+/H+ problem 20 VI. Ionizing radiation from hot gas-cool gas interfaces 20 A. Types of interfaces 21 B. A test case 21 VII. Some questions for future study 22 A. What is the source of the elevated temeratures? 22 B. What is the spatial distribution of the gas? 23 C. How much ionizing radiation escapes the galaxy? 23 arXiv:0901.0941v1 [astro-ph.GA] 7 Jan 2009 2 D. Do hot, pre-white dwarf stars play a role? 23 E. Is missing atomic data important? 23 F. What insights will new global models provide? 23 Acknowledgments 24 A list of acronyms and terms 24 References 25 I. INTRODUCTION Composed primarily of hydrogen (91% by number) and helium (9%), with trace amounts (0.1%) of heavier ele- ments, the interstellar medium plays a vital role in the cycle of stellar birth and death and galactic evolution. Not only do the properties of the interstellar medium govern the formation of new stars, but through their ra- diation and the matter and kinetic energy from their out- flows and supernovae, these stars in turn determine the properties of the interstellar medium from which the next generation of stars will be born. One of these feedback processes, the subject of this review, is the large-scale ionization of the medium by the youngest and most lu- minous stars, the O stars. Even though they are located near the galactic midplane in rare, isolated regions of star formation and often surrounded by opaque clouds of neutral hydrogen, the Lyman continuum radiation from these hot stars is somehow able to propagate large dis- tances through the disk and into the galaxy’s halo to produce extensive ionization of the interstellar hydrogen. The study of this wide-spread plasma has impacted our understanding of the dynamic interstellar processes oc- curring in galaxies. This area of study began more than four decades ago, when Hoyle and Ellis (1963) proposed to a skeptical as- tronomical community the existence of an extensive layer of warm (104 K), low density (10−1 cm−3) ionized hy- drogen surrounding the plane of our Galaxy and having a power requirement comparable to the ionizing luminos- ity of the Galaxy’s O and B stars. Their conclusion was based upon their discovery of a free-free absorption signa- ture in the observations of the Galactic synchrotron back- ground at frequencies between 1.5 and 10 MHz carried out by radio astronomy pioneers Grote Reber and G. R. A. Ellis at Hobart, Tasmania (Ellis et al

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