M 31 - the Andromeda Galaxy - is the nearest spiral galaxy on the sky and it was the first external galaxy detected in radio continuum (Hanbury Brown & Hazard 1950). Surveys with the Effelsberg telescope and with the Cambridge and Westerbork synthesis instruments revealed a ring-like structure at about 10 kpc radius (Pooley 1969; Berkhuijsen & Wielebinski 1974; Beck & Gräve 1982) and a large number of point-like sources (Bystedt et al. 1984; Walterbos et al. 1985). The northern half of M 31 was mapped with the VLA by Braun (1990) who detected some extended emission at a resolution of 90 using a special code to account for the missing spacings.
Polarized radio emission from M 31 was first detected by Beck et al. (1978, 1980) with the Effelsberg 100-m telescope at 11 cm. The magnetic field structure can be modeled by a torus of field lines with uniform direction without radial reversals (Beck 1982; Ruzmaikin et al. 1990) consistent with dynamo theory (Poezd et al. 1993). However, these results were based on modeling the distribution of polarization angles and polarized intensity at only one wavelength.
In an Effelsberg survey at 6 cm significant polarized emission was detected in the bright "ring'' within 30 of the minor axis and within 5 south of the centre (Berkhuijsen et al. 1987). Together with VLA observations at 20 cm of a region in the "ring'' southwest of the centre the torus model of the magnetic field was basically confirmed, but in the "ring'' also periodical field structures on smaller spatial scales were discovered (Beck et al. 1989).
This paper presents a complete VLA survey of M 31 at 20 cm of the total emission and, for the first time, the linearly polarized emission, combined with low-resolution observations of the total emission with the Effelsberg telescope at the same wavelength.