The proximity of M31 enables to study its central region with high linear resolution. Within 5' from the nucleus (about 1 kpc in the plane of M31) deep observations in the H line (Ciardullo et al., 1988) revealed a complex structure of narrow filaments on the northwestern side, a small bar-like structure and a spiral arm on the southeastern side. Ciardullo et al. found evidence for an outflow of gas from the inner 200 pc. Radio continuum observations at 49 cm and 21 cm with the Westerbork telescope showed extended emission from the central spiral and the filaments (Walterbos & Gräve, 1985). More recently, the central 20' was observed with the VLA at 6 cm and 20 cm (Hoernes, 1997) with angular resolutions of 13" and 22", respectively, corresponding to 45 pc and 150 pc in the plane of M31. The distribution of the total emission at 6 cm is very similar to that of the H emission (Devereux et al., 1994), with enhanced emission from the spiral arm in the SE and the brightest filaments in the NW. These results indicate field compression by large-scale shocks, either by a wind or by density waves.
The existence of synchrotron-emitting cosmic-ray electrons in the central region is puzzling as star-forming activity is weak there. Hoernes (1997) derived the distribution of the total spectral index (S ) between 20 cm and 6 cm with a resolution of 22"; it has a filamentary and patchy appearance. At the very centre the spectrum is flat with -0.2 and it steepens outwards. Hoernes (1997) showed that this variation cannot be explained by thermal emission. Hence, the nonthermal spectral index nt at the center must be close to -0.2; it slowly decreases along the southern arm and the filaments, but perpendicular to these features it decreases much faster, reaching values -1.0 at 1 kpc radius. This behaviour is similar to that observed in the central regions of the Milky Way (Pohl et al., 1992) and M81 (Reuter & Lesch, 1996) and suggests the existence of a black hole associated with a mono-energetic source of relativistic electrons in the nucleus (Hoernes et al., 1998b).
Polarized emission at 6 cm (Fig. 5) is weak in the NW region of the central filaments which is probably due to strong Faraday depolarization, but is concentrated on and around the southern spiral arm with the highest degree of polarization along the inside of the arm. This result is in favour of a density wave moving faster than the arm and compressing the gas and field at the inner edge.
Figure 5. Polarized intensity from the central region of M31, observed with the VLA and smoothed to 22" beamsize. The vectors lengths are proportional to the polarized intensities, their orientations have not been corrected for Faraday rotation (Hoernes, 1997).