3.5. Magnetic fields in external galaxies

An excellent presentation of the evidence of cosmic magnetism in nearby galaxies has been given by Beck et al. [15] and, in a more concise form, by Widrow [63]. The past few years have witnessed a long discussion on the primordial nature of magnetic fields in nearby galaxies whose features are partially reported in [15]. In analogy with the problem discussed in the case of the MW two different opinions have been confronted. Theorists believing the primordial nature of the galactic magnetic field supported the BSS model. On the contrary, dynamo theorists supported the conclusion that most of the nearby galaxies are ASS. As previously described, the variation of the RM as a function of the azimuthal angles has been used in order to distinguish ASS from BSS [64, 65]. The simplistic summary of the recent results is that there are galaxies where BSS structure dominates (like M81, M51) [66]. At the same time in some galaxies the ASS dominates [67].