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Date and Time of the Query: 2019-08-18 T00:45:20 PDT
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For refcode 1989A&A...217...17K:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1989A&A...217...17K The magnetic field structures in two nearby spiral galaxies II. The bisymmetric spiral magnetic field in M81. M. Krause, R. Beck, and E. Hummel Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-5300 Bonn 1, Federal Republic of Germany Received September 5, accepted September 29, 1988 Summary. We present new high-resolution observations of the total and linearly polarized emission of M81 at {lambda}20cm obtained with the VLA. Even at this low frequency we found a high degree of linear polarization of 47+/-3% in the southwestern interarm region. Hence, the degree of uniformity B_u_/B_t_ in this region is as high as 0.8+/-0.2. Depolarization considerations could remove the ambiquity of the rotation measure (RM) between {lambda}20cm and {lambda}6.3cm in this region. The RM analysis to determine the large-scale magnetic field structure in the disk of spiral galaxies is extended to derive the pitch angle and the location of the neutral line in the case of a bisymmetric spiral magnetic field structure (BSS). We show, for the first time, that the azimuthal RM distribution is clearly double-periodical between 6kpc and 12kpc radius. This gives strong evidence for a BSS magnetic field structure in M81 as generated by an {alpha}{omega}-dynamo with mode m=1. From the amplitude, the zerolevel and the phases of the two RM distributions we could conclusively derive a mean pitch angle {PSI}_0_ of the magnetic field spiral of -18^deg^+/-6^deg^. There are indications that {PSI}_0_ increases with radius. For the above value of {PHI}_0_ we found that the neutral lines are located approximately in the interarm regions. The high degree of uniformity in the southwestern interarm region indicates that the uniform magnetic field generated and/or maintained by the dynamo is less disturbed in the interarm region. Alternatively, the {alpha}{omega}-dynamo may work more effectively just in the interarm region due to density waves which modify the shear motion in a galaxy. Key words: spiral galaxies: M81 - interstellar magnetic field - radio continuum - polarization - Faraday rotation - Faraday depolarization
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