Issue |
A&A
Volume 487, Number 1, August III 2008
|
|
---|---|---|
Page(s) | 197 - 203 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:200809808 | |
Published online | 04 June 2008 |
The coexistence of odd and even parity magnetic fields in disc galaxies
1
School of Mathematics, University of Manchester, Oxford Road, Manchester, M13 9PL, UK e-mail: moss@ma.man.ac.uk
2
Department of Physics, Moscow University, 119992 Moscow, Russia
Received:
19
March
2008
Accepted:
30
May
2008
Aims. Naive dynamo models predict that large-scale magnetic fields generated in flattened disc-like structures will be steady and symmetric with respect to the equatorial plane, whereas fields generated in quasi-spherical volumes will be oscillatory and anti-symmetric. Spiral galaxies consist of a flattened disc and a quasi-spherical halo. We thus investigate to what extent this naive understanding of symmetry properties is realised in composite disc/halo models for galactic magnetic fields.
Methods. We consider
generation of galactic magnetic fields in the framework of galactic
mean field dynamo theory, based on the effects of differential
rotation and helical turbulent motions (the “α-effect”),
using conventional profiles for both generators of magnetic field in
the disc and halo. The halo and disc regions are mostly separated by
a substantial contrast between their turbulent diffusivities,
respectively and halo
. We solve the corresponding
equations of mean field electrodynamics numerically, using contrasts
up to
, while realizing that it might be realistic
to consider significantly larger values.
Results. In contrast to our naive expectations coexisting steady symmetric (quadrupole-like) magnetic structures in the disc and oscillating antisymmetric (dipole-like) structures in the halo were not found. Usually one component of the dynamo system enslaves the other: a more dynamo-active disc creates a symmetric field in the halo as well as in the disc or, conversely, a more dynamo-active halo generates antisymmetric magnetic fields that pervade both halo and disc. Our most interesting models are mixed parity solutions at the transition between the two regimes.
Conclusions. We consider the results obtained as presenting a challenge for the
contemporary theory of galactic magnetic fields. We note that there
is some recent observational evidence for a difference in symmetry
properties between disc and halo. We see three possible resolutions
of the problem. The contrast used may have been too
low (values up to about 50 can be argued for but implementation of
such values is rather demanding numerically). Another option is that
galactic magnetic structures may be separated in two distinct
classes, namely objects with disc-like and halo-like magnetic field
structures. To settle this issue observationally seems to require
much higher resolution than is possible with telescopes now
available. Finally, of course, can not be excluded that something
fundamental is lacking from our understanding of the origin of
galactic magnetic fields.
Key words: galaxies: magnetic fields / Galaxy: disk / ISM: magnetic fields
© ESO, 2008
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