Issue |
A&A
Volume 470, Number 2, August I 2007
|
|
---|---|---|
Page(s) | 539 - 556 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20066988 | |
Published online | 25 May 2007 |
Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes, and helical fields
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: rbeck@mpifr-bonn.mpg.de
Received:
20
December
2006
Accepted:
8
May
2007
Context.The spiral galaxy NGC 6946 hosts magnetic spiral arms, highly aligned magnetic fields between the gas/optical arms.
Aims.The origin of the magnetic phenomena and their relation to the interstellar gas are investigated.
Methods.NGC 6946 was observed in total intensity and linear polarization in five radio bands between 3 cm and 21 cm. Maps of spectral index, Faraday rotation and depolarization were derived.
Results.At the inner edge of the inner gas spiral arm the ordered magnetic
field is only mildly compressed and turns smoothly, to become aligned
along the gas arm. Hence the field is not shocked and is probably
connected to the warm, diffuse gas. At larger radii,
two bright magnetic arms between the optical arms are visible in
polarized intensity. The field in the northern magnetic arm is almost
totally aligned. Faraday rotation measures (RM) in
these arms are consistent with the superposition of two low
azimuthal dynamo modes. Three more magnetic arms are
discovered in the outer galaxy, located between arms.
The RM structure function confirms large-scale coherent fields.
The observed anti-correlation between the field's pitch angles and the RM
values is a possible signature of helical fields. –
Due to strong Faraday depolarization the galaxy is not
transparent to polarized waves at
and
. The
large-scale asymmetry in depolarization with respect to the major
axis may be another indication of large-scale helical fields.
Three depolarization rings of almost zero polarization seen at
are probably generated by differential Faraday rotation in
complexes in NGC 6946 of 300-500 pc size. –
In the gas/optical spiral arms, the total (mostly turbulent) magnetic
field is amplified to
20 μG. Its energy density is
10
times larger than that of the ionized gas and is similar to that of the
turbulent gas motions in the inner galaxy. The magnetic energy exceeds
that of the turbulent energy in the outer galaxy. All energy densities
in NGC 6946 are about one order of magnitude larger than those in the
Milky Way.
Conclusions.Density waves in the inner gaseous spiral arms mildly compress the field. Dynamo action probably generates the magnetic spiral arms. The magnetic field is dynamically important, interacts with the gas flow and possibly determines the properties of the gas spiral arms.
Key words: galaxies: spiral / galaxies: magnetic fields / galaxies: ISM / galaxies: individual: NGC 6946 / radio continuum: galaxies / radio continuum: ISM
© ESO, 2007
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