2021PASJ...73.1429I

We study accretion environments of active galactic nuclei when a supermassive black hole wanders in a circumnuclear region and passes through an interstellar medium there. It is expected that 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 recent studies, the accretion ring consists of a thick envelope and a thin core, and angular momenta are transferred from the inner side facing the black hole to the opposite side in the envelope and the core respectively. 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 extend 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 supersonic wind flowing to 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 bipolar cones. These pictures provide a likely guideline to interpreting the overall accretion environments suggested from observations.