Accretion environments of active galactic nuclei
  We study accretion environments of active galactic nuclei when a
super-massive black hole wanders in a circum-nuclear region and passes through
an interstellar medium there. It is expected that a Bondi-Hoyle-Lyttleton type
accretion of the interstellar matter takes place and an accretion stream of
matter trapped by the black hole gravitational field appears from a tail shock
region. Since the trapped matter is likely to have a certain amount of specific
angular momentum, the accretion stream eventually forms an accretion ring
around the black hole. According to the recent study, the accretion ring
consists of a thick envelope and a thin core, and angular momenta are
transfered from the inner side facing to the black hole to the opposite side
respectively in the envelope and the core. As a result, a thick accretion flow
and a thick excretion flow extend from the envelope, and a thin accretion disk
and a thin excretion disk do from the core. The thin excretion disk is
predicted to terminate at some distance forming an excretion ring, while the
thick excretion flow is considered to become a super-sonic wind flowing to the
infinity. The thick excretion flow from the accretion ring is expected to
interact with the accretion stream toward the accretion ring and to be
collimated to bi-polar cones. These pictures provide a likely guide line to
interpret the overall accretion environments suggested from observations.
