On rapid migration and accretion within discs around supermassive black holes

Galactic nuclei should contain a cluster of stars and compact objects in the vicinity of the central supermassive black hole due to stellar evolution, minor mergers and gravitational dynamical friction. By analogy with protoplanetary migration, nuclear cluster objects (NCOs) can migrate in the accretion discs that power active galactic nuclei (AGN) by exchanging angular momentum with disc gas. Here we show that an individual NCO undergoing runaway outward migration comparable to type III protoplanetary migration can generate an accretion rate corresponding to Seyfert AGN or quasar luminosities. Multiple migrating NCOs in an AGN disc can dominate traditional viscous disc accretion, and at large disc radii, ensemble NCO migration and accretion could provide sufficient heating to prevent the gravitational instability from consuming disc gas in star formation. The magnitude and energy of the X-ray soft excess observed at ∼0.1-1 keV in Seyfert AGN could be explained by a small population of ∼10²-10³ accreting stellar mass black holes or a few ULXs. NCO migration and accretion in AGN discs are therefore extremely important mechanisms to add to realistic models of AGN discs.