Spiral arm structures revealed in the M31 galaxy
  Striking regularities are found in the northwestern arm of the M31 galaxy.
Star complexes located in this arm are all spaced 1.2 kpc apart and have
similar sizes of about 0.6 kpc. Within the same arm region Beck et al. (1989)
detected a regular magnetic field, and we found that its wavelength is the
spacing between the complexes. In this arm, groups of HII regions lie inside
star complexes, which, in turn, are located inside the gas and dust lane. In
contrast, the southwestern arm of M31 splits into a gas and dust lane upstream
and a dense stellar arm downstream, with HII regions located mostly along the
boundary between these components of the arm. The stellar density in the
southwestern arm is much higher than in the northwestern arm, and the former is
not fragmented into star complexes. The age gradient across this arm have been
found in earlier observations. According the classical SDW theory, these
drastic differences may be due due to their different pitch angles: about 0
degree for northwestern part of the arm and about 30 degree for the
southwestern segment. Data on M31, M51, M74, and some other galaxies suggest
that star complexes are mostly located in the arm segments that are not
accompanied by a dust lane upstream, i.e. do not host spiral shock wave. The
regularities in the distribution of complexes along a density wave spiral arm
are most probably due to the development of the Parker-Jeans instability, which
builds up star complexes if the initial SFR in the arm is low and, as a
sequence, magnetic field is regular along the arm.
