Polarized synchrotron radiation from the Andromeda Galaxy M31 and
  background sources at 350 MHz
  Polarization measurements at low radio frequencies allow detection of small
Faraday rotation measures caused by regular magnetic fields in galaxies and in
the foreground of the Milky Way. The galaxy M31 was observed in two overlapping
pointings with the Westerbork Synthesis Radio Telescope (WSRT) resulting in ~4'
resolution in total intensity and linearly polarized emission. The frequency
range 310-376 MHz was covered by 1024 channels which allowed the application of
RM synthesis. We derived a data cube in Faraday depth and compared two
symmetric ranges of negative and positive Faraday depths. This new method
avoids the range of high instrumental polarization and allows the detection of
very low degrees of polarization. For the first time, diffuse polarized
emission from a nearby galaxy is detected below 1 GHz. The degree of
polarization is only 0.21 +/- 0.05 %, consistent with extrapolation of internal
depolarization from data at higher radio frequency. A catalogue of 33 polarized
sources and their Faraday rotation in the M31 field is presented. Their average
depolarization is DP(90,20) = 0.14 +/- 0.02, 7 times stronger depolarized than
at 1.4 GHz. We argue that this strong depolarization originates within the
sources, e.g. in their radio lobes, or in intervening galaxies on the line of
sight. On the other hand, the Faraday rotation of the sources is mostly
produced in the foreground of the Milky Way and varies significantly across the
~9 square degree M31 field. As expected, polarized emission from M31 and
extragalactic background sources is much weaker at low frequencies compared to
the GHz range. Future observations with LOFAR, with high sensitivity and high
angular resolution to reduce depolarization, may reveal diffuse polarization
from the outer disks and halos of galaxies.
