Lecture Notes in Physics

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Cosmic Magnetic Fields

ABC Editors

Richard Wielebinski Rainer Beck Max-Planck-Institut für Radioastronomie Auf dem Hügel 69 53121 Bonn Germany E-mail: [email protected] E-mail: [email protected]

Richard Wielebinski and Rainer Beck, Cosmic Magnetic Fields, Lect. Notes Phys. 664 (Springer, Berlin Heidelberg 2005), DOI 10.1007/b104621

Library of Congress Control Number: 2005930474

ISSN 0075-8450 ISBN-10 3-540-24175-2 Springer Berlin Heidelberg New York ISBN-13 978-3-540-24175-1 Springer Berlin Heidelberg New York

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Typesetting: by the author using a Springer LATEX macro package Printed on acid-free paper SPIN: 11369875 55/Techbooks 543210 Preface

Recent years have seen great progress in the field of cosmic magnetic fields, both in the available observational data and in the theoretical interpretations. Now we have extensive data on magnetic fields in virtually all types of cosmic objects: the Earth, the Sun, the Milky Way, out to magnetic fields in the most distant reaches of the Universe. The first detection of a magnetic field outside the Earth was made by G.E. Hale, who made optical Zeeman effect observations of magnetic fields in the Sun, nearly 100 years ago. The detection of the Zeeman effect in stars by H.W. Babcock followed some 40 years later. Optical starlight polariza- tion observations were made in 1949 and, when combined with the Davis– Greenstein effect interpretation, suggested that magnetic fields were present in our Milky Way. Radio polarization measurements confirmed this deduc- tion in 1962. With developing sensitivity of radio and optical observations magnetic fields have been shown to be present practically everywhere. We know that the Sun is driven by magnetic fields. Supernova remnants show us the evolution of the magnetic field in shock fronts that follow a stellar explo- sion. and X-ray sources have been shown to possess extremely intense magnetic fields. The Milky Way is a magnetic laboratory, with complex mag- netic field structures, worthy of exploration. Regular patterns of large-scale magnetic fields are observed in nearby galaxies and radio galaxies. Also clus- ters of galaxies were shown to be permeated with detectable magnetic fields. In spite of this mounting evidence about the presence of magnetic fields in the cosmic Universe only a few attempts have been made to interpret the situations taking into account all the relevant parameters, in particular those resulting from the presence of magnetic fields. In many publications elaborate numerical investigations are carried out but without the consideration of the action of a magnetic field. Only recently did we get magneto-hydrodynamic codes. These developments show us that the inclusion of magnetic fields is indeed necessary. Possibly the whole approach to the interpretation of obser- vational data will change as a result of magnetic effects. The next decade in will shed more light on the role of magnetic fields in the Uni- verse. We should soon find out if the magnetic fields are only a consequence of gas motion, or are they at the heart of the matter? VI Preface

The scheme of this book is to start with the most distant magnetic fields in the Universe and then make our way back to our Galaxy. Contributions based on observational data will describe magnetic fields in clusters of galax- ies, in radio galaxies, nearby galaxies and in the Milky Way. Contributions on various theoretical effects and considerations of the magnetic fields in the Uni- verse are interspersed between the observational chapters where appropriate. The review articles in this book do not cover all aspects of cosmic magnetic fields. In particular no contribution on solar magnetic fields has been included since in recent years numerous reviews and books have covered this subject adequately. In the first contribution Martin Rees considers the magnetic fields in the early Universe. A substantial magnetic field could have been generated in the early Universe, however no relevant physics to this early stage of our development has been proposed do far. It is argued that the first significant magnetic fields must have been formed in the course of formation of the early non-linear structures giving the needed seed fields needed for the dynamo action. The build-up of the magnetic fields is an important aspect of the cosmogonic process. The second contribution by Philipp Kronberg deals with the magnetic fields in galaxy systems, clusters of galaxies and beyond. The history of the realization that cosmic rays and magnetic fields are essential for the observed radio emission is given. This chapter focuses on the possible role of stars, black holes and supernovae, in injecting magnetic energy into the intergalactic medium. These two broad categories of energy output from galaxies have been recognized for some time. The discussion begins with the environment of galaxies out to a distance of the local Supercluster of galaxies and proceeds all the way back to the formation of first stars and galaxies. The magnetic effect in radio galaxies are also considered. Rainer Beck’s contribution describes the magnetic fields in nearby galax- ies. There has been a tremendous progress in this field in recent years. This progress in observations reveals a wide range of large-scale magnetic phenom- ena. Spiral fields exist in grand-design and flocculent galaxies, and even some dwarf galaxies host ordered fields. Regular magnetic fields trace the gas flow in barred galaxies. Vertical magnetic fields observed above disks of edge-on galaxies indicate strong galactic winds into the halos. Magnetic fields possi- bly help to feed the active galactic nuclei, which may solve a long-standing problem. A contribution by Russel Kulsrud on the origin of Galactic magnetic field follows. From considerations of the origin of cosmic rays the existence of magnetic field became obvious. Assuming that a magnetic field did exist there is no problem in sustaining it. On the other hand there are problems to create such a magnetic field. The discussion touches on the well known alpha–omega disc dynamo as well as evolution of primordial magnetic fields. Arguments for and against either interpretation are clearly given, pointing Preface VII out that the question of the origin of the galactic magnetic fields remains open. In the fifth contribution Richard Wielebinski describes the present knowl- edge about the magnetic fields of the Milky Way, derived from radio contin- uum and Faraday effect observations. The basics of the synchrotron emission theory are sketched showing their application to the observations of magnetic fields. The development of radio observations is given and the latest results described. A combination of radio polarization surveys and Faraday Rotation Measure studies of pulsars and extragalactic radio sources are expected to lead to a ‘tomography’, a three-dimensional description of the magnetic field of the Milky Way. The following contribution by Anvar Shukurov on the mesoscale magnetic structure in spiral galaxies attempts to give interpretation for the observa- tions described in Chapters 3 and 5. Various observed phenomena, like regular magnetic fields in the inter-arm regions of spiral galaxies or the observed field reversal in the Milky Way are considered. It is shown that the dynamo theory has been impressively successful in explaining the gross features of galactic magnetic fields. Systematic studies of structures on intermediate scales should advance our understanding on the nature of cosmic magnetism. The seventh contribution by Carl Heiles and Richard Crutcher deals with the details of magnetic fields in diffuse H I and molecular clouds. The Zee- man effect allows direct measurement of the magnetic fields in these objects and the recent increase in sensitivity gave us a large number of new results. Polarization of starlight is discussed as a basic phenomenon that delineates the local magnetic fields. Polarization of thermal grains is sketched as well as spectral-line polarization. The history of H I Zeeman observations is dis- cussed in detail as well as the development of gathering the data on molecular lines. Although a large volume of reliable data has been collected the future developments, like the new large radio telescopes, are described since they hold a key for future new results. In the contribution that follows Leon Mestel and John Landstreet give us an overview on the state of stellar polarization observations. Stellar magnetic fields have been detected across the whole Hertzsprung–Russell diagram. This contribution concentrates on pre-main sequence and on late- and early-type stars, with some discussion of red giants and white dwarfs. The current ob- servational situation is reviewed and some consequences on star formation, stellar structure and evolution are summarized. The two final contributions deal with details of some theoretical aspects of magnetic fields. Axel Brandenburg considers the importance of helicity in dynamo theory. Magnetic helicity is conserved and its evolution provides a dynamical feedback on the alpha effect that is distinct from alpha quenching. The explicit connection with catastrophic alpha quenching is reviewed and the alleviating effects of magnetic and current helicity fluxes are discussed. VIII Preface

The final chapter by Max Camenzind deals with numerical magnetohy- dodynamics (MHD) in astrophysics. In the past 10 years powerful numerical algorithms and computational methods have been developed for simulating the time evolution of magnetic fields in astrophysical environments. The most recent trends go to fully conservative schemes and adaptive mesh refinement for large-scale supercomputing. Examples of such computer simulations are given. A discussion of General Relativistic MHD codes, still in development, is given. The collection of contributions for this book needed more than one year. Some authors used this time to update their manuscripts in view of the quick changes in this subject. In this time also concrete plans for new instruments (ALMA, LOFAR, SKA) have been crystallizing. ‘Cosmic Magnetism’ is one of the Key Science Projects for the Square Kilometer Array (SKA). Great steps forward in our understanding of magnetic fields will be possible. The editors wish to thank Gabi Breuer for adapting all the contributions to the Springer macro system. We wish to thank Anton Zensus for the support that made this book a reality.

Bonn Richard Wielebinski February, 2005 Rainer Beck Contents

Magnetic Fields in the Early Universe Martin J. Rees ...... 1 1 Introduction ...... 1 2 MagneticFieldsfromtheUltra-EarlyUniverse?...... 2 3 Protogalactic Batteries ...... 4 4 MagneticFieldsfromtheFirstStars...... 5 5 AGNsandRadioLobes...... 6 6 Summary...... 7 References ...... 7 Magnetic Fields in Galaxy Systems, Clusters and Beyond Philipp P. Kronberg ...... 9 1 Introduction and Background ...... 9 2 Stellar Sources of Extragalactic Magnetic Fields ...... 10 3 Early Plasma-driven Seeding Scenarios ...... 14 4 The Galactic Large-scale α − Ω DynamoTheory ...... 14 5 Proxy Measurements for Cosmologically Early, Star-ejected MagneticFields...... 15 6 Post-amplification of Initially Weak Intergalactic Seed Fields of AllKinds ...... 16 7 Massive Black Hole Seeding of Intergalactic Magnetic Fields ...... 17 8 Gravitational Collapse and Black Hole Electromagnetic Energy Generators ...... 21 9 Radio Galaxies as ‘Calorimeters’ ofBH-injectedMagneticFieldsandCR’s...... 24 10 Magnetic Fields in Clusters of Galaxies ...... 26 11 Probes of Magnetic Fields at Larger Redshifts, Beyond Galaxies andClusters...... 32 12 Summary...... 37 References ...... 37 X Contents

Magnetic Fields in Galaxies Rainer Beck ...... 41 1 Introduction ...... 41 2 Observing Extragalactic Magnetic Fields ...... 41 3 MeasuringMagneticFieldStrengths...... 43 4 MagneticFieldsandGasClouds ...... 45 5 MagneticFieldStructure...... 47 6 Dynamos ...... 53 7 Magnetic Fields in Flocculent and Irregular Galaxies ...... 55 8 Magnetic Fields in Barred Galaxies ...... 56 9 Halos...... 59 10 Interacting Galaxies ...... 61 11 Spiral Galaxies with Jets ...... 63 12 Outlook...... 64 References ...... 65 The Origin of Galactic Magnetic Fields Russel M. Kulsrud ...... 69 1 Introduction ...... 69 2 TheAlpha–OmegaDiscDynamo...... 70 3 Evolution of a Primordial Magnetic Field ...... 75 4 The Protogalactic Dynamo ...... 82 5 Conclusion ...... 87 References ...... 88 Magnetic Fields in the Milky Way, Derived from Radio Continuum Observations and Faraday Rotation Studies Richard Wielebinski ...... 89 1 Introduction ...... 89 2 Observational Rationale ...... 90 3 The Earliest Observations ...... 93 4 RadioContinuumSurveysoftheMilkyWay...... 94 5 Surveys of Rotation Measure of Extragalactic Radio Sources (EGRS) ...... 102 6 Pulsars as Probes of the Magnetic Fields of the Galaxy ...... 104 7 TheMagneticFieldsoftheMilkyWay...... 105 References ...... 108 Contents XI

Mesoscale Magnetic Structures in Spiral Galaxies Anvar Shukurov ...... 113 1 Introduction ...... 113 2 Observational Evidence for Magnetic Reversals ...... 114 3 GlobalReversals...... 117 4 LocalizedReversals...... 123 5 MagneticArms ...... 125 6 Conclusions...... 133 References ...... 133

Magnetic Fields in Diffuse H I and Molecular Clouds Carl Heiles, Richard Crutcher ...... 137 1 Introduction ...... 137 2 Measuring the Magnetic Field in Diffuse H I andMolecularClouds...... 139 3 Observed vs. Intrinsic Probability Density Functions ...... 146 4 B|| from H I Absorption Lines ...... 153 5 B|| from H I EmissionLines...... 157 6 Importance of Magnetic Fields in Molecular Clouds ...... 163 7 Molecular Cloud Observational Results ...... 164 8 Model Predictions and Observational Tests ...... 170 9 Magnetic Field Observations, Present and Future ...... 176 References ...... 179 Stellar Magnetic Fields Leon Mestel, John D. Landstreet ...... 183 1 Stellar Magnetism ...... 183 2 Magnetism and Star Formation ...... 188 3 Pre-main Sequence Stars: Observation and Theory ...... 191 4 The Main-sequence: Late-type Stars ...... 193 5 The Main Sequence: Early-type Stars ...... 200 6 The Early-type Magnetic Stars: Basic Theoretical Questions ...... 207 7 GiantStars...... 211 8 Degenerate Stars ...... 212 References ...... 213 XII Contents

Importance of Magnetic Helicity in Dynamos Axel Brandenburg ...... 219 1 Introduction ...... 219 2 Dynamos in a Periodic Box ...... 221 3 Magnetic Helicity Evolution ...... 223 4 What Do Stars and Galaxies Do Differently? ...... 228 5 Connection with the α Effect ...... 235 6 What about η Quenching? ...... 244 7 Conclusions...... 250 References ...... 251 Numerical Magnetohydrodynamics in Astrophysics Max Camenzind ...... 255 1 Introduction ...... 255 2 The Classical MHD Model in Computer Simulation ...... 256 3 Progress in Understanding Fundamental MHDProcesses...... 263 4 Special Relativistic MHD (SRMHD) Limits oftheClassicalMHD...... 270 5 Relativistic MHD for Rotating Black Holes (GRMHD) ...... 273 6 FutureProspects...... 277 References ...... 278 List of Contributors

Martin J. Rees Anvar Shukurov Institute of Astronomy School of Mathematics Madingley Road and Statistics Cambridge, CB3 0HA, UK University of Newcastle [email protected] Newcastle upon Tyne NE1 7RU, U.K. [email protected] Philipp P. Kronberg Institute of Geophysics and Planetary Physics Carl Heiles Los Alamos National Laboratory Astronomy Department NM 87501, USA University of California [email protected] Berkeley, CA 94720, USA [email protected]

Rainer Beck Richard Crutcher Max-Planck-Institut Astronomy Department f¨ur Radioastronomie University of Illinois Auf dem H¨ugel 69 Urbana, IL 61801, USA 53121 Bonn, Germany [email protected] [email protected]

Leon Mestel Russel M. Kulsrud Astronomy Centre Plasma Physics Laboratory Falmer, Brighton Princeton NJ 08544, USA BN1 9QH, England [email protected] [email protected]

Richard Wielebinski John D. Landstreet Max-Planck-Institut Department of Physics f¨ur Radioastronomie and Astronomy Auf dem H¨ugel 69 University of Western Ontario 53121 Bonn, Germany London, ON N6A 3K7, Canada [email protected] [email protected] XIV List of Contributors

Axel Brandenburg Max Camenzind Nordita, Blegdamsvej 17 Landessternwarte K¨onigstuhl 2100 Copenhagen Ø Denmark 69117 Heidelberg, Germany [email protected] [email protected]