The nuclear stellar disc in Andromeda: a fossil from the era of black hole growth

The physics of angular momentum transport from galactic scales (similar to 10-100 pc) to much smaller radii is one of the outstanding problems in our understanding of the formation and evolution of supermassive black holes (BHs). Seemingly unrelated observations have discovered that there is a lopsided stellar disc of unknown origin orbiting the BH in M31, and possibly many other systems. We show that these nominally independent puzzles are in fact closely related. Multiscale simulations of gas inflow from galactic to BH scales show that when sufficient gas is driven towards a BH, gravitational instabilities form a lopsided, eccentric disc that propagates inwards from larger radii. The lopsided stellar disc exerts a strong torque on the remaining gas, driving inflows that fuel the growth of the BH and produce quasar-level luminosities. The same disc can produce significant obscuration along many sightlines and thus may be the putative 'torus' invoked to explain obscured active galactic nuclei and the cosmic X-ray background. The stellar relic of this disc is long lived and retains the eccentric pattern. Simulations that yield quasar-level accretion rates produce relic stellar discs with kinematics, eccentric patterns, precession rates and surface density profiles in reasonable agreement with observations of M31. The observed properties of nuclear stellar discs can thus be used to constrain the formation history of supermassive BHs.