Magnetic Fields of Galaxies

321 Pages·1988·6.327 MB·English

by  A. A. Ruzmaikin, A. M. Shukurov, D. D. Sokoloff (auth.)

MAGNETIC FIELDS OF GALAXIES
ASTROPHYSICS AND
SPACE SCIENCE LIBRARY
A SERIES OF BOOKS ON THE RECENT DEVELOPMENTS
OF SPACE SCIENCE AND OF GENERAL GEOPHYSICS AND ASTROPHYSICS
PUBLISHED IN CONNECTION WITH THE JOURNAL
SPACE SCIENCE REVIEWS
Editorial Board
R. L. F. BOYD, University College, London, England
W. B. BURTON, Sterrewacht, Leiden, The Netherlands
L. GOLDBERG, Kitt Peak National Observatory, Tucson, Ariz., U.S.A.
C. DE JAGER, University of Utrecht, The Netherlands
J. KLECZEK, Czechoslovak Academy of Sciences, Ondrejov, Czechoslovakia
Z. KOPAL, University ofM anchester, England
R. LUST, European Space Agency, Paris, France
L. I. SEDOV, Academy ofS ciences of the U.S.S.R., Moscow, U.S.S.R.
Z. SVESTKA, Laboratory for Space Research, Utrecht, The Netherlands
VOLUME 133
A. A. RUZMAIKIN, A. M. SHUKUROV, AND D. D. SOKOLOFF
Space Research Institute, Academy of Sciences,
Moscow. U.S.S.R.
MAGNETIC
FIELDS OF
GALAXIES
Kluwer Academic Publishers
Dordrecht I Boston / London
Library of Congress Cataloging· in· Publication Data
Ruzmaikin, A, A, (Aleksandr Andreevich)
Magnetic fields of galaxies.
(Astrophysics and space science library; v. 133)
Bibliography: p.
Includes index.
1. Galaxies - Magnetic fields. I. Shukurov, A, M., 1949- II. Sokoloff,
O. O. III. Title. IV. Series.
QB857.5.M34R89 1987 523.1 '12 87-36917
ISBN· 13: 978·94·010·7776·7 e· ISBN· 13: 978·94·009·2835·0
001: 10.1007/978·94·009·2835·0
Published by Kluwer Academic Publishers,
P.O. Box 17,3300 AA Dorclrecht, The Netherlands.
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All rights reserved.
© 1988 by Kluwer Academic Publishers
Softcover reprint of the hardcover 1s t edition 1988
No part of the material protected by this copyright notice may be reproduced or
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TABLE OF CONTENTS
PREFACE vii
CHAPTER 1/ INTRODUCTION 1
CHAPTER II / GALAXIES 7
ILl. Shapes 7
11.2. Spiral Galaxies 11
II.3. Gas and Dust 12
11.4. Cosmic Rays 17
CHAPTER III / OBSERVATION OF MAGNETIC FIELDS 20
IlL 1. Synchrotron Emission 20
IIL2. Polarization of Synchrotron Emission 24
IlI.3. Faraday Rotation 32
IlI.4. Light Polarization by Dust 37
IlL5. Zeeman Splitting 47
IlI.6. Other Methods 53
III.7. Discussion 54
CHAPTER IV / INTERPRETATION OF OBSERVATIONAL DATA 57
IV.1. The Magic of Data Processing 57
IV.2. How the Magnetic Field is Derived from Faraday Rotation
Data 64
IV.3. The Large-scale Magnetic Field of the Galaxy According to
Faraday Rotations of Extragalactic Sources 71
IV.4. The Fluctuation Magnetic Field in the Galaxy 79
IV.5. The Structure of the Large-Scale Field 80
IV6. Intensity Variations of the Galactic Non-thermal Radio
Background 81
IV7. Magnetic Fields in Nearby Spiral Galaxies 85
CHAPTER V / ORIGIN OF MAGNETIC FIELDS 95
V.l. Introduction 95
V2. The Relic Field Hypothesis 97
V3. Cosmological Magnetic Fields 103
V.4. Stellar Ejections 109
v
vi TABLE OF CONTENTS
V.5. The Dynamo 113
CHAPTER VI/GALACTIC HYDRODYNAMICS 122
VI.l. Rotation 122
VI.2. Shape of the Gaseous Disc 133
VI.3. Interstellar Turbulence 142
VIA. Mean Helicity 162
VI.5. Magnetic Fields and Star Formation 166
CHAPTER VII / THE GALACTIC DYNAMO 171
VII.1. Introduction 171
VIl.2. The Mean Magnetic Field 172
VII.3. Evolution of Magnetic Field in a Moving Medium 173
VITA. The Equation for the Mean Magnetic Field 177
VII.5. Field Distribution Across the Disc 181
VII.6. Radial Field Distribution in Discs of Variable Thickness 1 91
VII.7. Radial Distribution of Axisymmetric Fields in Spiral
Galaxies 200
VII.8. Generation of Non-Axisymmetric Magnetic Fields in an
Axisymmetric Disc 209
VII.9. The Origin of Large-Scale Bisymmetric Magnetic Structures 224
VILlO. Large-Scale Magnetic Fields in Rigidly Rotating Objects 233
VILl1. Magnetic Fields Within Spiral Arms 242
VII.12. Non-linear Effects in the Galactic Dynamo 246
VII.13. Generation of Fluctuation Fields 248
VII.14. Seed Fields 255
CHAPTER VIII / MAGNETIC FIELDS AROUND GALACTIC DISCS 261
VIII.1. Magnetic Fields in Gaseous Coronae 261
VIII. 2. Magnetic Fields in Clusters of Galaxies 264
CHAPTER IX / PROBLEMS OF MAGNETIC FIELD GENERATION IN
GALACTIC NUCLEI. QUASARS AND RADIO GALAXIES 269
IX. 1. Center of the Galaxy 269
IX.2. Quasars and Active Galactic Nuclei 271
IX.3. Radiogalaxies 274
IXA. Jets 278
CHAPTER X / CONCLUDING REMARKS 284
REFERENCES 286
INDEX 311
PREFACE
Magnetism, when extended beyond normal frameworks into cosmic space is
characterized by an enormous spatial scale. Because of their large sizes the nature
of magnets such as the Earth and the Sun is entirely different from the nature of a
horseshoe magnet. The source of cosmic magnetism is associated with the
hydrodynamic motions of a highly conductive medium. In this aspect, cosmic
magnets resemble a dynamo. However, currents in the dynamo flow along
properly ordered wires, while chaotic, turbulent motions are dominant inside stars
and liquid planetary cores. This makes more intriguing and surprising the fact that
these motions maintain a regular magnetic field.
Maintenance of magnetic fields is even more impressive in huge magnets, i.e.
galaxies. In fact, we are living inside a giant dynamo machine, the Milky Way
galaxy. Although the idea of the global magnetic field of our Galaxy was clearly
proposed almost 40 years ago, firm observational evidence and definite theoretical
concepts of galactic magnetism have been developed only in the last decade. This
book is the first attempt at a full and consistent presentation of this problem. We
discuss both theoretical views on the origin of galactic magnetism and the methods
of observational study. Previous discussions were on the level of review articles or
separate chapters in monographs devoted to cosmic magnetic fields (see, e.g., H.
K. Moffatt, 1978, E. N. Parker, 1979 and Zeldovich et aI., 1983).
Some readers will expect to see a detailed discussion of the popular problems
of quasars and radiogalaxies. However, one can only touch upon them, asking
some questions concerning the origin and maintenance of magnetic fields in these
objects. In this book we would like to unveil the new depth, fascination and
novelty which have been discovered in those places where they were hardly
expected: well known spiral galaxies such as the Andromeda Nebula and the
Whirlpool galaxy. Indeed, recently discovered and studied global structures of
magnetic fields in spiral galaxies which resemble rings and two-armed spirals are
very impressive.
Our subject is not "monumentum aere perennius". The field of galactic
magnetism is very young and is developing rapidly. Many questions are far from
being answered, many ideas are disputed. However, the information already
available deserves to be discussed in a monograph rather than a review paper.
The authors do not intend to hide their theoretical background. On the one
hand, the principal concept of galactic magnetism as a product of a hydromagnetic
vii
viii PREFACE
dynamo belongs to theoreticians. On the other hand, after personal encounter with
observational data processing and after numerous discussions with those who see a
radiotelescope every morning we recognize and appreciate even more the impor
tance of observational efforts and hope that this feeling is reflected in this book.
We would like to thank those who have helped us, directly or indirectly. Among
these are Yulia Baryshnikova, Rainer Beck, V. L. Ginzburg, S. A. Molchanov, R.
Wielebinski, and Ya. B. Zeldovich. Special thanks are due to Rita Bujakaite,
Natalia Furdina, Natalia Gula, Nina A. Sokolova and Karina Ter-Saakova for their
help in the preparation of the manuscript.
CHAPTER I
INTRODUCTION
Magnetic fields in galaxies have a modest strength, which is conveniently measured
in microgauss. Galactic magnetic fields are notable for another property: a huge
spatial scale. This is expressed in kiloparsecs. These magnets, having an enormous
scale but still much neglected, are the subject of our book.
Six out of every ten observed galaxies are spirals and a major proportion of
recent observational and theoretical results in the investigation of galactic magnetic
fields have been obtained for spiral galaxies. It is therefore not accidental that our
attention mainly concerns spirals and primarily the Galaxy, our birthplace.
The first views on galactic magnetic fields began being assembled shortly after
the Second World War. In those years, thorough investigation of cosmic rays
began at the same time as the construction of accelerators for elementary particle
research. In those years the radio telescope became the peacetime brother of radar.
It had been noted even before the War (see, e.g., Alfven, 1937) that electro
magnetic fields within stars, near stars and in interstellar space could have strength
and scale sufficient to accelerate charged particles up to energies observed in
cosmic ray experiments. In 1949, E. Teller, R. Richtmeier and H. Alfven proposed
and advocated the idea that cosmic rays have a solar origin and are confined near
the Sun by an interplanetary magnetic field. Directions of motions of primary
cosmic rays that are presented by protons, electrons, a-particles and a small
quantity of heavy nuclei, are distributed very isotropically. The radius r(cm) -
10/300 ZH of trajectory of a relativistic particle with the charge Ze and energy
e(eV) in the magnetic field H(G) should not exceed the scale of the planetary
system. Richtmeier and Teller (1949) therefore concluded that the magnetic field
H "" 10-6 G is required for confinement of particles with 10 "" 1014 eV in the
region of the size r "" 3 X 1017 cm. Such a field would keep particles with the
energy 1017 eV in a region comparable to the thickness of the Galactic disc,
3 X 1020 cm. The magnetic field can be amplified from some initial weak field by
motions of a highly conductive interstellar medium. If the magnetic field is
amplified until it reaches the energy equipartition with motions, H2/8:rc = pv2/2, it
would have the strength H "" 3 X 10-6 G for the interstellar gas density p =
10-24 g cm-3 and typical gas velocity v = 10 km S-I (Alfven, 1949).
E. Fermi (1949) did not agree with the hypothesis of the local, solar origin of
cosmic rays and suggested that they fill at least the whole Galaxy. This suggestion
naturally implies the existence of magnetic fields at still larger scales. Fermi noted
2 CHAPTER I
that vast expanses of interstellar space and the high conductivity of interstellar
gas imply a remarkable stability of Galactic magnetic fields. On the other
hand, moving inhomogeneities of the magnetic field can effectively accelerate the
particles.
In these early debates, true pictures were formed of the strength and scale of the
Galactic magnetic field.
Impressive evidence of the interstellar magnetic field then came from the
discovery of the polarization of starlight (Hall and Mikesell, 1949; Hiltner,
1949a, b). Electric vectors of the light coming from distant stars are orientated
closely to the Galactic plane. The fairly coherent distribution of polarization planes
suggests a universal origin of the orientation of dust particles which scatter and
absorb the starlight (for instance, the Davis-Greenstein mechanism).
Astronomers have noticed imprints of magnetic fields in the morphological
peculiarities of gas distribution in the Galaxy. The founder of the Crimean
Astrophysical Observatory, G. Shajn suggested in 1955 that elongation of diffuse
nebulae along the galactic plane is due to the large-scale interstellar magnetic field.
Nonthermal radioernission of the Galaxy was interpreted as a synchrotron
emission, i.e. magneto bremsstrahlung of relativistic electrons gyrating in the mag
netic field (Alfven and Herlofson, 1950; Kiepenheuer, 1950; Ginzburg, 1953;
Shklovsky, 1956). The detection of polarized radioemission in the Galaxy (Razin,
1958; Westerhout et al., 1962; Wielebinski et al., 1962) supported these earlier
theoretical predictions. From those times until now, synchrotron radioemission has
provided an efficient way to investigate interstellar magnetic fields in our Galaxy
and in external radiosources, too. This emission originates in extended regions of
the intercloud medium and thereby demonstrates the existence of the general
Galactic magnetic field.
However, these were indirect items of evidence. Galactic magnetic fields have
remained a hypothetical object. Of paramount importance, thus, was a direct
detection of the Zeeman splitting of the A 21 cm absorption line of atomic
hydrogen in gas clouds projected onto strong radio sources. This detection was an
enormously difficult problem. The splitting ~ v = eHlmc ~ 10 Hz should be
detected on the background of Doppler line broadening which is about 104 Hz at
T = 102 K. According to Verschuur (1979), the first five successful detections
have consumed 5000 hours of telescope time! The idea behind the detection of the
Zeeman effect is based on comparison of intensities of right- and left-circularly
polarized wings of the absorption line (Bolton and Wild, 1957). This method has
allowed the detection of magnetic fields in several neutral hydrogen clouds in the
Galaxy (Verschuur, 1968). However, the clouds have much higher densities than
the interstellar gas on average and the fields within them are a result of compres
sion and distortion of the Galactic magnetic field by a cloud.
The most effective indicator of the large-scale Galactic magnetic field has
proved to be the Faraday rotation of the polarization plane of radioernission from