Origin of Galactic and Extragalactic Magnetic Fields

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in s

Origin of Galactic and Extragalactic Magnetic FieldsLawrence M. WidrowDepartment of Physics, Queen’s University, Kingston, Ontario, Canada K7L 3N6A variety of observations suggest that magnetic elds are present in all galaxies and galaxy clusters. These elds are characterized by a modest strength (10 7 10 5 G) and huge spatial scale (< ～ 1 Mpc). It is generally assumed that magnetic elds in spiral galaxies arise from the combined action of di erential rotation and helical turbulence, a process known as the αω-dynamo. However fundamental questions concerning the nature of the dynamo as well as the origin of the seed elds necessary to prime it remain unclear. Moreover, the standard αω-dynamo does not explain the existence of magnetic elds in elliptical galaxies and clusters. The author summarizes what is known observationally about magnetic elds in galaxies, clusters, superclusters, and beyond. He then reviews the standard dynamo paradigm, the challenges that have been leveled against it, and several alternative scenarios. He concludes with a discussion of astrophysical and early Universe candidates for seed elds.arXiv:astro-ph/0207240v1 11 Jul 2002ContentsI. INTRODUCTION II. Preliminaries A. Magnetohydrodynamics and Plasma Physics B. Cosmology III. Observations of Cosmic Magnetic Fields A. Observational Methods 1. Synchrotron Emission 2. Faraday rotation 3. Zeeman Splitting 4. Polarization of Optical Starlight B. Spiral Galaxies 1. Field Strength 2. Global Structure of the Magnetic Field in Spirals 3. Connection with Spiral Structure 4. Halo Fields 5. Far Infrared-Radio Continuum Correlation C. Elliptical and Irregular Galaxies D. Galaxy Clusters E. Extracluster Fields F. Galactic Magnetic Fields at Intermediate Redshifts G. Cosmological Magnetic Fields 1. Faraday Rotation due to a Cosmological Field 2. Evolution of Magnetic Fields in the Early Universe 3. Limits from CMB Anisotropy Measurements 4. Constraints from Big Bang Nucleosynthesis 5. Intergalactic Magnetic Fields and High Energy Cosmic Rays IV. Galactic and Extragalactic Dynamos A. Primordial Field Hypothesis B. Mean-Field Dynamo Theory C. Disk Dynamos D. Growth Rate for the Galactic Magnetic Field E. Criticisms of Mean-Field Dynamo Theory F. Numerical Simulations of Disk Dynamos G. Diversity in Galactic Magnetic Fields H. Variations on the Dynamo Theme 1. Parker Instability 2. Magnetorotational Instability 3. Cross-Helicity Dynamo I. Dynamos in Irregular and Elliptical Galaxies and Galaxy Clusters 1. Elliptical Galaxies 2. Clusters V. Seed Fields 2 3 3 6 6 7 7 8 9 10 10 10 11 12 14 15 15 16 17 18 18 19 22 22 23 24 25 25 27 29 32 32 34 36 37 37 38 38 39 40 40 412A. Minimum Seed Field for the Galactic Dynamo B. Astrophysical Mechanisms 1. Seed Fields from Radiation-Era Vorticity 2. Biermann Battery E ect 3. Galactic Magnetic Fields from Stars 4. Active Galactic Nuclei C. Seed Fields from Early Universe Physics 1. Post-in ation Scenario

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength (10^{-7}-10^{-5} G) and huge spatial scale (~Mpc). It is generally assumed that magnetic fields in s

s 2. In ation-Produced Magnetic Fields VI. Summary and Conclusions Acknowledgments References 41 43 44 45 47 48 49 50 52 53 54 55I. INTRODUCTIONThe origin of galactic and extragalactic magnetic elds is one of the most fascinating and challenging problems in modern astrophysics. Magnetic elds are detected in galaxies of all types and in galaxy clusters whenever the appropriate observations are made. In addition there is mounting evidence that they exist in galaxies at cosmological redshifts. It is generally assumed that the large-scale magnetic elds observed in disk galaxies are ampli ed and maintained by an αω-dynamo wherein new eld is regenerated continuously by the combined action of di erential rotation and helical turbulence. By contrast, the magnetic elds in non-rotating or slowly rotating systems such as elliptical galaxies and clusters appear to have a characteristic coherence scale much smaller than the size of the system itself. These elds may be generated by a local, turbulent dynamo where, in the absence of rapid rotation, the eld does not organize on large scales. In and of itself, the dynamo paradigm must be considered incomplete since it does not explain the origin of the initial elds that act as seeds for subsequent dynamo action. Moreover, the timescale for eld ampli cation in the standard αω-dynamo may be too long to explain the elds observed in very young galaxies. It is doubtful that magnetic elds have ever played a primary role in shaping the large-scale properties of galaxies and clusters. In present-day spirals, for example, the energy in the magnetic eld is small as compared to the rotation energy in the disk. To be sure, magnetic elds are an important component of the interstellar medium (ISM) having an energy density that is comparable to the energy density in cosmic rays and in the turbulent motions of the interstellar gas. In addition, magnetic elds can remove angular momentum from protostellar clouds allowing star formation to proceed. Thus, magnetic elds can play a supporting role in the formation and evolution of galaxies and clusters but are probably not essential to our understanding of large-scale structure in the Universe. The converse is not true: An understanding of structure formation is paramount to the problem of galactic and extragalactic magnetic elds. Magnetic elds can be created in active galactic nuclei (AGN), in the rst generation of stars, in the shocks that arise during t …… 此处隐藏：7568字，全部文档内容请下载后查看。喜欢就下载吧 ……