GALACTIC MAGNETISM: Recent Developments and Perspectives

Abstract

We discuss current observational and theoretical knowledge of magnetic fields, especially the large-scale structure in the disks and halos of spiral galaxies. Among other topics, we consider the enhancement of global magnetic fields in the interarm regions, magnetic spiral arms, and representations as superpositions of azimuthal modes, emphasizing a number of unresolved questions. It is argued that a turbulent hydromagnetic dynamo of some kind and an inverse cascade 1 Now at Department of Mathematics and Statistics, University of Newcastle upon Tyne, NE1 7RU, United Kingdom. 155 0066-4146/96/0915-0155$08.00 156 BECK ET AL of magnetic energy gives the most plausible explanation for the regular galactic magnetic fields. Primordial theory is found to be unsatisfactory, and fields of cosmological origin may not even be able to provide a seed field for a dynamo. Although dynamo theory has its own problems, the general form of the dynamo equations appears quite robust. Finally, detailed models of magnetic field generation in galaxies, allowing for factors such as spiral structure, starbursts, galactic winds, and fountains, are discussed and confronted with observations.

Origin of Cosmic Magnetic Fields

“…After magnetic fields had been predicted to exist in the disk of our Galaxy inside the interstellar medium on the basis of cosmic ray arguments, [2,3,27], and the presence of optical polarization from dust scattering, [64], with a strength predicted to be around 5 microGauss, to within 20 percent, they were indeed detected, with a strength as expected, see, e.g., [1]. Now the best measurement for the Solar neighborhood is 6 -7 microGauss, with about 1/3 to 1/2 regular, and 2/3 to 1/2 irregular components, [1].…”

“…Recent reviews are in [33,25,1,30,27,28]. Large scale magnetic fields are discussed in [72,29,56,7,26,8,6,73].…”

Primordial large-scale electromagnetic fields from gravitoelectromagnetic inflation

“…The contribution to the magnetic fields is exclusive from the homogeneous solution in (26), this should be clear from (6) because the source terms are all gradients. The k-power that come from the homogeneous solution of the modes is m − 1 2 .…”

“…The existence, strength and structure of these fields in the intergalactic plane, within the Local Superclusted, has been scrutinized recently [2]. Many spiral galaxies are endowed with coherent magnetic fields of µG (micro Gauss) strength [3,4,5,6,7,8], having approximately the same energy density as the Cosmic Microwave Background Radiation (CMBR). In particular, the field strength of our galaxy is B ≃ 3 × 10 −6 G, similar to that detected in high redshift galaxies [9] and damped Lyman alpha clouds [10].…”

Electroweak Baryogenesis and Primordial Hypermagnetic Fields

“…Estimation of magnetic fields strengths -by synchrotron emission and Faraday rotation-require the independent estimation or assumption of the local electron density and the spatial structure of the field. Both quantities are reasonably known for our galaxy, where the average field strength is measured to be 3 − 4µG; various spiral galaxies in our neighborhood present fields that are homogeneous over galactic sizes, with similar magnetic field intensities [31,32]. At larger scales, only model dependent upper limits can be established.…”