Mesoscale Magnetic Structures in Spiral Galaxies

Abstract

Virtually all spiral galaxies host magnetic fields ordered at scales comparable to the galactic size (Beck et al., 1996; Beck, 2000, 2001). Observations of polarized radio emission at improved resolution and sensitivity have revealed details of the global magnetic structures that can shed new light on the problem of their origin. Reversals of the regular magnetic field along radius and/or azimuth and magnetic arms are such features, whose scale exceeds significantly the correlation scale of interstellar turbulence but remains smaller than the overall galactic dimension. Despite a few decades of debate, there remains doubt as to what features of the observed field could have been inherited from the pre-galactic past, and which have been formed and maintained more recently in a relatively mature galaxy. In what follows, we briefly review the current understanding of the origin of the mesoscale magnetic structures and their implications for the origin of galactic magnetic fields. The Milky Way appears to possess a global magnetic field of unusual structure. The regular magnetic field in our Galaxy has one or more large-scale reversals, where the magnetic field coherent over a scale of order a few kiloparsecs changes its direction by about 180◦ along a line presumably extended along the azimuth. The number of reversals has not been firmly established, their origin has not been fully understood, and the shape of the lines along which the reversals occur is not known. There are just a few galaxies where similar large-scale reversals cannot be excluded. The nearby galaxy M81 might host a bisymmetric magnetic structure (Krause et al., 1989), i.e., a global structure where the regular spiral magnetic field reverses along azimuth and, perhaps, radius. However, the magnetic structure of M81 needs to be reconsidered with observations at higher resolution and sensitivity and with more reliable interpretation techniques. A magnetic reversal between the inner and outer regions in the galaxy NGC 2997 has been suggested in Han et al. (1999). A magnetic reversal in the disc of M51 (Berkhuijsen et al., 1997) is discussed in detail below. The unusual structure of the Galactic magnetic field has attracted significant attention. Numerous papers have been published attempting to establish the number of reversals and their positions from observations. However, there is only a handful of papers where the origin of the magnetic reversals is addressed. In this review, we discuss some limitations of the observational evidence for the reversals and put the observational effort into a broader physical perspective of the theory of galactic magnetic fields.