A Supernova-driven, Magnetically-collimated Outflow as the Origin of the
  Galactic Center Radio Bubbles
  A pair of non-thermal radio bubbles recently discovered in the inner few
hundred parsecs of the Galactic center bears a close spatial association with
elongated, thermal X-ray features called the X-ray chimneys. While their
morphology, position, and orientation vividly point to an outflow from the
Galactic center, the physical processes responsible for the outflow remain to
be understood. We use three-dimensional magnetohydrodynamic simulations to test
the hypothesis that the radio bubbles/X-ray chimneys are the manifestation of
an energetic outflow driven by multiple core-collapsed supernovae in the
nuclear stellar disk, where numerous massive stars are known to be present. Our
simulations are run with different combinations of two main parameters, the
supernova birth rate and the strength of a global magnetic field being
vertically oriented with respect to the disk. The simulation results show that
a hot gas outflow can naturally form and acquire a vertically elongated shape
due to collimation by the magnetic pressure. In particular, the simulation with
an initial magnetic field strength of 80 $\mu$G and a supernova rate of 1
$kyr^{-1}$ can well reproduce the observed morphology, internal energy and
X-ray luminosity of the bubbles after an evolutionary time of 330 kyr. On the
other hand, a magnetic field strength of 200 $\mu$G gives rise to an overly
elongated outflow that is inconsistent with the observed bubbles. The
simulations also reveal that, inside the bubbles, mutual collisions between the
shock waves of individual supernovae produce dense filaments of locally
amplified magnetic field. Such filaments may account for a fraction of the
synchrotron-emitting radio filaments known to exist in the Galactic center.
