Issue |
A&A
Volume 398, Number 3, February II 2003
|
|
---|---|---|
Page(s) | 937 - 948 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20021710 | |
Published online | 28 January 2003 |
The polarized disk in M 31 at λ6 cm
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Corresponding author: E. M. Berkhuijsen, eberkhuijsen@mpifr-bonn.mpg.de
Received:
8
August
2002
Accepted:
7
November
2002
We present a radio continuum survey of M 31 at λ6.2 cm
with high sensitivity in total power and polarization, observed with
the 100-m Effelsberg dish with an angular resolution of 2 4.
(1) Combination with the VLA + Effelsberg map at λ20.5 cm in
total power yielded a spectral index map at
resolution. Both
the spectrum of the integrated emission and the spectral index
distribution across M 31 indicate a nonthermal spectral index
. We derived maps of thermal and
nonthermal emission at λ6.2 cm. Radial profiles of the various
emission components north and south of the minor axis revealed that the
stronger total emission in the northern part of M 31 is entirely due
to stronger thermal emission, whereas the profiles of nonthermal and
polarized emission are nearly identical on either side of the minor
axis. This suggests that recent star formation does not lead to a local
increase of the number of relativistic electrons and/or magnetic field
strength.
(2) We discuss several properties of the polarized emission from
M 31 and their implications for the magnetic field. At λ6.2 cm
the polarized intensity systematically varies along the bright “ring” of
emission which shows that the regular magnetic field,
,
is nearly aligned with the spiral arms forming this “ring”. The
variation of the rotation measures between λ11.1 cm and
λ6.2 cm, RM(11, 6), across the galaxy confirms this
alignment. The nonthermal degree of polarization reaches values
>
near the polarization maxima, implying that the magnetic
field in M 31 is exceptionally regular.
(3) We derived the distribution of the nonthermal depolarization
between λ11.1 cm and λ6.2 cm,
, which is a
measure of Faraday depolarization. Gradients in RM(11, 6) may be an
important cause of Faraday depolarization in M 31. The lack of
anticorrelation between the thermal emission, which comes mainly from
dense
regions with small filling factors, and RM(11, 6) and
indicates that rotation measures and Faraday
depolarization originate in the extended diffuse ionized gas.
Key words: galaxies: individual: M 31 / galaxies: spiral / galaxies: magnetic fields / ISM: magnetic fields / radio continuum: galaxies / polarization
© ESO, 2003
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