ENHANCED ACCRETION RATES OF STARS ON SUPERMASSIVE BLACK HOLES BY STAR–DISK INTERACTIONS IN GALACTIC NUCLEI

We investigate the dynamical interaction of a central star cluster surrounding a supermassive black hole (SMBH) and a central accretion disk (AD). The dissipative force acting on stars in the disk leads to an enhanced mass flow toward the SMBH and to an asymmetry in the phase space distribution due to the rotating AD. The AD is considered as a stationary Keplerian rotating disk, which is vertically extended in order to employ a fully self-consistent treatment of stellar dynamics including the dissipative force originating from star-gas ram pressure effects. The stellar system is treated with a direct high-accuracy N-body integration code. A star-by-star representation, desirable in N-body simulations, cannot be extended to real particle numbers yet. Hence, we carefully discuss the scaling behavior of our model with regard to particle number and tidal accretion radius. The main idea is to find a family of models for which the ratio of two-body relaxation time and dissipation time (for kinetic energy of stellar orbits) is constant, which then allows us to extrapolate our results to real parameters of galactic nuclei. Our model is derived from basic physical principles and as such it provides insight into the role of physical processes in galactic nuclei, but it should be regarded as a first step toward more realistic and more comprehensive simulations. Nevertheless, the following conclusions appear to be robust: the star accretion rate onto the AD and subsequently onto the SMBH is enhanced by a significant factor compared to purely stellar dynamical systems neglecting the disk. This process leads to enhanced fueling of central disks in active galactic nuclei (AGNs) and to an enhanced rate of tidal stellar disruptions. Such disruptions may produce electromagnetic counterparts in the form of observable X-ray flares. Our models improve predictions for their rates in quiescent galactic nuclei. We do not yet model direct stellar collisions in the gravitational potential well of the black hole, which could further enhance the growth rate of the black hole. Our models are relevant for quiescent galactic nuclei, because all our mass accretion rates would give rise to luminosities much smaller than the Eddington luminosity. To reach Eddington luminosities, outflows, and feedback as in the most active QSOs, other scenarios are needed, such as gas accretion after galaxy mergers. However, for AGNs close to the Eddington limit, this process may not serve as the dominant accretion process due to the long timescale.