Bisymmetric Spiral Magnetic Fields in Spiral Galaxies

Abstract

A radio polarization analysis was presented in 1978 by TOSA and FUJIMOTO [1] to determine a large-scale configuration of magnetic fields in the spiral galaxy M51. On the basis of the distributions of planes of polarization at 6 cm and 21 cm [2], they determined the rotation measures (RMs) of the Faraday effect and the intrinsic polarization angles. The former gives the line-of-sight directions of magnetic fields and the latter the orientations projected onto the plane of sky. Figure 1 shows the latter result for M51. We are impressed that the projected field lines are not closed but spiral and parallel to the luminous arms. Another method was proposed for distinguishing between the circular and the bisymmetric spiral magnetic fields by measuring the variation of RM or the angle between the polarization planes at 6 and 21 cm along a concentric circle in the M51 disk. If the magnetic field is circular, we have one maximum and one minimum in RM along the circle from 0 to 2π (Fig. 2a), and if it is bisymmetric spiral, we have two minima and two maxima (Fig. 2b). Figure 3 suggests that M51 is the latter case rather than the former. See [4,5] and papers presented at this workshop for the BSS fields observed in more detail and extensively in other galaxies.