Annu. Rev. Astron. Astrophys. 1996. 34: 155-206 Copyright © 1996 by Annual Reviews. All rights reserved

3. MAGNETIC FIELDS IN SPIRAL GALAXIES

3.1. Field Strengths

Mean equipartition strengths of the total field <B^3+_s>^{1 / (3+_s)} (averaged over the volume of the visible radio disk) range from 4 µG in M33 (Buczilowski & Beck 1991) to 12 µG in NGC 6946 and NGC 1566 Ehle & Beak, Ehle et al 1996); they are proportional to surface brightness in the far-infrared range (Hummel et al 1988b) and to average gas density (S Niklas et al, in preparation) Hummel's (1986) sample of 88 Sbc galaxies has a mean minimum-energy field of 8 µG, using K = 100. Using the same value of K for the sample of 146 late-type galaxies by Fitt & Alexander (1993), one obtains a mean total minimum-energy field strength of 10 ± 4 µG. Extremal values found in normal galaxies can be up to 20 µG in spiral arms, as in NGC 6946 (Beck 1991) and NGC 1566 (Ehle et al 1996). In the mildly active galaxy M82, Klein et al (1988) found a field strength 50 µG.

The regular field strengths as obtained from the intensity of polarized emission are typically a few µG. Such values are roughly consistent with regular field strengths _|| as derived from Faraday rotation data, if we assume typical electron densities of a few 10^-2 cm^-3 (see e.g. Buczilowski & Beck 1991). Because polarized intensity and rotation measure depend differently on the filling factor of the field, the fact that _|| implies that the filling factor is not very small. The ratio of regular to turbulent field strengths is typically 0.5 if observed with a spatial resolution of a few kpc (Buczilowski & Beck 1991).

In NGC 2276 the regular field strength reaches 10 µG (Hummel & Beck 1995), probably due to its interaction with the ambient intracluster gas. The total field is also unusually strong in that galaxy. Galaxies in clusters generally contain stronger fields (Gavazzi et al 1991, Niklas et al 1995).