In "The Interstellar Medium in M31 and M33",
Proc. 232. WE-Heraeus-Seminar, 22-25 May 2000, Bad Honnef,
Germany. Edited by Elly M. Berkhuijsen, Rainer Beck, and Rene
A. M. Walterbos. Shaker, Aachen, 2000, p. 171-178.
astro-ph/0009455
For a PDF version of the article, click
here.
RADIO CONTINUUM EMISSION FROM M31 AND M33
Rainer Beck
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69, 53121 Bonn, Germany
Abstract.
The radio emission from M31 (like HI, CO, FIR and
H
) is
concentrated in the "10 kpc ring", giving an impressive example
that cosmic rays are produced in star-forming regions.
M31 and M33 have similar strengths of the total
magnetic field, but very
different field structures: The field structure in M31 is exceptionally
regular while that in M33 is rather irregular compared with other
spiral galaxies. In M33 the polarized intensity is highest between the
spiral arms, similar to most spiral galaxies, while in M31 total and
polarized emission both emerge from the ring. Star formation
in M31 is probably too weak to tangle the regular field.
The high regularity of the field in the M31 ring allows fast cosmic-ray
propagation. As a consequence, there is no equipartition between
the energy densities of cosmic rays and total magnetic fields.
Faraday rotation measures show that the regular field in the ring is
uni-directional, signature of the basic axisymmetric dynamo mode
with a pitch angle of only -12°.
Faraday rotation of polarized background sources shows that the
regular field and thermal gas in M31 extend to at least 25 kpc radius.
The regular field in M33 forms an open spiral, a mixture of axisymmetric
and higher modes, with the largest pitch angle
(
60°) observed
in any spiral galaxy so far.
Vertical filaments in the NW and SE indicate interaction between
the thin and the thick disk of M31. The total emission in
the central region of M31 follows the spiral and radial
H
filaments, while the polarized emission is strongest on the inner edge of
the southern spiral filament.
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