Javier Alfaro / Perez / Pérez How does the Galaxy work?

A Galactic Tertulia with Don Cox and Ron Reynolds
1. Auflage 2006
ISBN: 978-1-4020-2620-1
Verlag: Springer Netherlands
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, Band 315, 426 Seiten

Reihe: Astrophysics and Space Science Library

This volume presents a multi-disciplinary approach to the physical mechanisms which structure the shape, evolution and fate of the Galaxy. It contains 4 panel sessions (the recording transcripts of the galactic 'tertulias'), an introductory historical overview on the Galaxy research, 17 reviews, and the proceedings of more than 50 oral and poster contributions.

This book is not a new edition of a previous volume but it is in some way related to the book "The Formation of the Milky Way" (1995, Cambridge University Press), which contains the proceedings of the first IAA-IAC-University of Pisa meeting, held in Granada ten years ago, on the formation of the Milky Way, (see Shore’s introduction).

Where many other books focus their attention on singular galactic components or specific driving forces, this title provides a wide overview on the gaseous and stellar components and on the physical mechanisms which maintain the dynamical equilibrium of such a complex system as the Galaxy. The book, intended for graduate students and researchers, provides a valuable overview on the different physical mechanisms which drive the galactic gas-stars feedback.

Written for: Graduate students, researchers

Autoren/Hrsg.

Pérez, Enrique

Franco, José

Javier Alfaro, Emilio

Perez, Enrique

Leseproben

MAGNETIC FIELDS IN THE MILKY WAY AND OTHER SPIRAL GALAXIES (p. 277-278)
R. Beck MPI für Radioastronomie, Germany

Abstract

The average strength of the total magnetic field in the Milky Way, derived from radio synchrotron data under the energy equipartition assumption, is 6µG locally and 10µG at 3 kpc Galactic radius. Optical and synchrotron polarization data yield a strength of the local regular field of 4µG (an upper limit if anisotropic fields are present), while pulsar rotation measures give 1.5µG (a lower limit if small-scale fluctuations in regular field strength and in thermal electron density are anticorrelated). In spiral arms of external galaxies, the total [regular] field strength is up to 35µG [ 15µG]. In nuclear starburst regions the total field reaches 50µG. Little is known about the global field structure in the Milky Way. The local regular field may be part of a "magnetic arm" between the optical arms, a feature that is known from other spiral galaxies. Unlike external galaxies, rotation measure data indicate several global field reversals in the MilkyWay, but some of these could be due to field distortions. The Galaxy is surrounded by a thick radio disk of similar extent as around many edge-on spiral galaxies. While the regular field of the local disk is of even symmetry with respect to the plane (quadrupole), the regular field in the inner Galaxy or in the halo may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane.


1. Motivation

Magnetic fields are a major agent in the interstellar medium. They contribute significantly to the total pressure which balances the ISM against gravitation. They affect the gas flows in spiral arms and around bars. Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud during its collapse. MHD turbulence distributes energy from supernova explosions within the ISM. Magnetic reconnection is a possible heating source for the ISM and halo gas. Magnetic fields also control the density and distribution of cosmic rays in the ISM.

2. Observing magnetic fields

Polarized emission at optical, infrared, submillimeter and radio wavelengths is the clue to interstellar magnetic fields. Radio continuum emission at centimeter wavelengths, emitted by cosmic-ray electrons in the interstellar medium, by pulsars and by background quasars, has higher degrees of polarization than in the other spectral ranges and provides the most extensive and reliable information on large-scale interstellar magnetic fields in our Galaxy (Heiles 1996, Han et al. 1999a) and about 70 external galaxies (see list in Beck 2000).

The observable degree of polarization is reduced by the contribution of unpolarized thermal emission which may dominate in star-forming regions, by Faraday depolarization (Sokoloff et al. 1998) and by geometrical depolarization within the beam. The orientation of polarization vectors is changed in a magneto-ionic medium by Faraday rotation which is generally small below about ?6 cm so that the B–vectors (i.e. the observed E–vectors rotated by 90.) directly trace the orientation of the regular (or anisotropic) fields in the sky plane. Polarization angles are ambiguous by ±180. and hence insensitive to field reversals. Compression or stretching of turbulent .elds generates incoherent anisotropic .elds which reverse direction frequently within the telescope beam, so that Faraday rotation is small while the degree of polarization can be high.