Investigating Magnetic Activity in the Galactic Centre by Global MHD
  Simulation
  By performing a global magnetohydrodynamical (MHD) simulation for the Milky
Way with an axisymmetric gravitational potential, we propose that spatially
dependent amplification of magnetic fields possibly explains the observed
noncircular motion of the gas in the Galactic centre (GC) region. The radial
distribution of the rotation frequency in the bulge region is not monotonic in
general. The amplification of the magnetic field is enhanced in regions with
stronger differential rotation, because magnetorotational instability and
field-line stretching are more effective. The strength of the amplified
magnetic field reaches >~ 0.5 mG, and radial flows of the gas are excited by
the inhomogeneous transport of angular momentum through turbulent magnetic
field that is amplified in a spatially dependent manner. As a result, the
simulated position-velocity diagram exhibits a time-dependent asymmetric
parallelogram-shape owing to the intermittency of the magnetic turbulence; the
present model provides a viable alternative to the bar-potential-driven model
for the parallelogram shape of the central molecular zone. In addition, Parker
instability (magnetic buoyancy) creates vertical magnetic structure, which
would correspond to observed molecular loops, and frequently excited vertical
flows. Furthermore, the time-averaged net gas flow is directed outward, whereas
the flows are highly time dependent, which would contribute to the outflow from
the bulge.
