2014A&A...571A..61G

The Andromeda Galaxy (M 31) is the nearest grand-design spiral galaxy. Thus far, most studies in the radio regime concentrated on the 10kpc ring. The central region of M 31 has significantly different properties than the outer parts: The star formation rate is low, and inclination and position angle are largely different from the outer disk. The existing model of the magnetic field in the radial range 6≤r≤14kpc is extended to the innermost part r≤0.5kpc to ultimately achieve a picture of the entire magnetic field in M 31. We combined observations taken with the VLA at 3.6cm and 6.2cm with data from the Effelsberg 100-m telescope to fill the missing spacings of the synthesis data. The resulting polarization maps were averaged in sectors to analyse the azimuthal behaviour of the polarized intensity (PI), rotation measure (RM), and apparent pitch angle (φ_obs). We developed a simplified 3D model for the magnetic field in the central region to explain the azimuthal behaviour of the three observables. Our 3D model of a quadrupolar or dipolar dynamo field can explain the observed patterns in PI, RM, and φ_obs, while a 2D configuration is not sufficient to explain the azimuthal behaviour. In addition and independent of our model, the RM pattern shows that the spiral magnetic field in the inner 0.5kpc points outward, which is opposite to that in the outer disk, and has a pitch angle of ≃33°, which is much larger than that of 8°-19° in the outer disk. The physical conditions in the central region differ significantly from those in the 10kpc ring. In addition, the orientation of this region with respect to the outer disk is completely different. The opposite magnetic field directions suggest that the central region is decoupled from the outer disk, and we propose that an independent dynamo is active in the central region.