Magnetohydrodynamic Simulations of a Plunging Black Hole into a
  Molecular Cloud
  Using two-dimensional magnetohydrodynamic simulations, we investigated the
gas dynamics around a black hole plunging into a molecular cloud. In these
calculations, we assumed a parallel-magnetic-field layer in the cloud. The size
of the accelerated region is far larger than the Bondi-Hoyle-Lyttleton radius,
being approximately inversely proportional to the Alfv\'en Mach number for the
plunging black hole. Our results successfully reproduce the "Y" shape in
position velocity maps of the "Bullet" in the W44 molecular cloud. The size of
the Bullet is also reproduced within an order of magnitude using a reasonable
parameter set. This consistency supports the shooting model of the Bullet,
according to which an isolated black hole plunged into a molecular cloud to
form a compact broad-velocity-width feature.
