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). |