Numerical simulation of large-scale magnetic-field evolution
in spiral galaxies
Katarzyna Otmianowska-Mazur
Astronomical Observatory, Jagiellonian University, ul. Orla 171,
Krakow, Poland
Max-Planck-Institut für Radioastronomie, Auf dem Hügel
69, D-53121 Bonn, Germany
Masashi Chiba
Max-Planck-Institut für Radioastronomie, Auf dem Hügel
69, D-53121 Bonn, Germany
Astronomical Institute, Tohoku University, Sendai 980-77, Japan
The evolution of large-scale magnetic fields in disk galaxies
is investigated numerically. The gasdynamical simulations in a
disk perturbed by spiral or bar potential are incorporated into
the kinematic calculations of induction equations to elucidate
the effects of non-axisymmetric disk structure on magnetic fields.
The effects of interstellar turbulence are given as the turbulent
diffusion of magnetic fields. The usually adopted dynamo mechanism
of $\alpha$-effect is not considered in our computations, because
it is not obvious about the actual existence of the effect in
a galaxy. Our principal concern is to clear how observationally
and theoretically well-established gas flow affects the
magnetic-field structure and evolution, without putting a lot
of artificial parameters in the model. We have found that the
density-wave streaming motion of gas has a significant influence
on the distribution of magnetic fields: the lines of force are
well aligned with spiral arms due to the compressional and additional
shearing flow of gas in these regions. In the inter-arm regions,
the field lines have the finite angles with respect to the imposed
arms, because the gaseous streamlines induced by spiral arms deviate
from the orientation of spiral arms themselves. These properties
of magnetic-field orientation across the arm are well in agreement
with the results of radio continuum observations. All simulation
models have resulted in the eventual decay of magnetic energy
due to the strong turbulent diffusion. We have also explored
the azimuthally periodic function for the coefficient of turbulent
diffusion with the maximum in spiral-arm regions. This is anticipated
from the enhancement of turbulence by young OB stars and supernovae.
This effect of non-uniform dissipation gives rise to the modulation
of magnetic-field structure and its time evolution. In particular,
the dissipation rate of magnetic fields can be much smaller than
the usually assumed rate of $\sim 1/10^8$ yr$^{-1}$.
Keywords - magnetic fields - MHD - galaxies: magnetic fields
- kinematics and dynamics - spiral