Magnetic Fields in the Milky Way and Other Spiral Galaxies

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 Milky Way, 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.

Dissecting the spiral galaxy M 83: mid-infrared emission and comparison with other tracers of star formation

“…This provides a relation between the number density N of gas clouds and field strength B of B 2 ∝ N. A similar relation is obtained through equipartition between the energy densities of turbulent cloud motion and magnetic fields, as e.g. observed in NGC 6946 (Beck 2004). The physical mechanism and time scale of this coupling has to be investigated.…”

An explicit representation for the axisymmetric solutions of the free Maxwell equations

“…Axially symmetric solutions of the Maxwell equations are quite important, at least as an often relevant approximation. For instance, axisymmetric magnetic fields occur naturally as produced by systems possessing an axis of revolution, such as disks or coils [1,2], or astrophysical systems like accretion disks [3] or disk galaxies [4]. Axisymmetric solutions are also used to model EM beams and their propagation (e.g.…”

Magnetic Fields in Irregular Galaxies

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Magnetic fields are an important component of the interstellar medium. They channel gas flows, accelerate and distribute energy from cosmic rays, and may be a significant component of the galactic pressure, especially in lowmass galaxies [1]. A priori, one wouldn't expect irregular galaxies to have large scale magnetic fields.

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“…The goal of this project is to significantly increase the number of irregular galaxies with observed magnetic field structures to better answer the following questions: (1) what generates and sustains large-scale magnetic fields in irregular galaxies? and (2) what causes the range of observed magnetic field structure?…”