Galactic Dark Matter in the Phantom Field
  We investigate the possibility that the galactic dark matter exists in an
scenario where the phantom field is responsible for the dark energy. We obtain
the statically and spherically approximate solution for this kind of the galaxy
system with a supermassive black hole at its center. The solution of the metric
functions is satisfied with $g_{tt} = - g_{rr}^{-1}$. Constrained by the
observation of the rotational stars moving in circular orbits with nearly
constant tangential speed in a spiral galaxy, the background of the phantom
field which is spatially inhomogeneous has an exponential potential. To avoid
the well-known quantum instability of the vacuum at high frequencies, the
phantom field defined in an effective theory is valid only at low energies.
Under this assumption, we further investigate the following properties. The
absorption cross section of the low-energy $S$-wave excitations of the phantom
field into the central black hole is shown to be the horizontal area of the
central black hole. Because the infalling phantom particles have a total
negative energy, the accretion of the phantom energy is related to the decrease
of the black hole mass which is estimated to be much less than a solar mass in
the lifetime of the Universe. Using a simple model with the cold dark matter
very weakly coupled to the "{\it low-frequency}" phantom particles which are
generated from the background, we show that these two densities can be
quasi-stable in the galaxy.
