Issue |
A&A
Volume 506, Number 3, November II 2009
|
|
---|---|---|
Page(s) | 1123 - 1135 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/200911698 | |
Published online | 27 August 2009 |
Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253*
II. The magnetic field structure
1
Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK e-mail: v.heesen@herts.ac.uk
2
Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany
3
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
Received:
21
January
2009
Accepted:
29
July
2009
Context. There are several edge-on galaxies with a known magnetic field structure in their halo. A vertical magnetic field significantly enhances the cosmic-ray transport from the disk into the halo. This could explain the existence of the observed radio halos.
Aims. We observed NGC 253 that possesses one of the brightest radio halos discovered so far. Since this galaxy is not exactly edge-on () the disk magnetic field has to be modeled and subtracted from the observations in order to study the magnetic field in the halo.
Methods. We used radio continuum polarimetry with the VLA in D-configuration
and the Effelsberg 100-m telescope. NGC 253 has a very bright nuclear
point-like source, so that we had to correct for instrumental
polarization. We used appropriate Effelsberg beam
patterns and developed a tailored polarization calibration to cope
with the off-axis location of the nucleus in the VLA primary
beams. Observations at 6.2 cm and 3.6 cm were combined to calculate the RM distribution and to correct for Faraday rotation.
Results. The large-scale magnetic field consists of a disk
and a halo
component. The disk component
can be described as an axisymmetric spiral field pointing inwards
with a pitch angle of
which is
symmetric with respect to the plane (even parity). This
field dominates in the disk, so that the observed magnetic field
orientation is disk parallel at small distances from the midplane.
The halo field shows a prominent X-shape centered on the nucleus
similar to that of other edge-on galaxies. We propose a model
where the halo field lines are along a cone with an opening angle
of
and are pointing away from the disk in both
the northern and southern halo (even parity). We can not exclude
that the field points inwards in the northern halo (odd
parity). The X-shaped halo field follows the lobes seen in
Hα and soft X-ray emission.
Conclusions. Dynamo action and a disk wind can explain the X-shaped halo field. The nuclear starburst-driven superwind may further amplify and align the halo field by compression of the lobes of the expanding superbubbles. The disk wind is a promising candidate for the origin of the gas in the halo and for the expulsion of small-scale helical fields as requested for efficient dynamo action.
Key words: galaxies: individual: NGC 253 / magnetic fields / methods: observational / methods: data analysis / galaxies: halos / galaxies: ISM
© ESO, 2009
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