The magnetic field structure of the central region in M31
  The Andromeda Galaxy (M31) is the nearest grand-design spiral galaxy. Thus
far most studies in the radio regime concentrated on the 10 kpc ring. The
central region of M31 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<=14 kpc is extended to the innermost part r<=0.5 kpc
to ultimately achieve a picture of the entire magnetic field in M31. We
combined observations taken with the VLA at 3.6 cm and 6.2 cm 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 (\phi_obs). We developed a simplified 3-D model for the
magnetic field in the central region to explain the azimuthal behaviour of the
three observables. Our 3-D model of a quadrupolar or dipolar dynamo field can
explain the observed patterns in PI, RM, and \phi_obs, while a 2-D
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 0.5 kpc points outward, which is opposite to that in the
outer disk, and has a pitch angle of about 33 degrees, which is much larger
than that of 8-19 degrees in the outer disk. The physical conditions in the
central region differ significantly from those in the 10 kpc 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.
