The Nuclear Stellar Disk in Andromeda: A Fossil from the Era of Black Hole Growth

TL;DR

This paper links the formation of eccentric stellar disks around supermassive black holes to gas inflow processes, showing that such disks are relics of black hole growth phases and can explain observed features in galaxies like Andromeda.

Contribution

It demonstrates through simulations that lopsided stellar disks are formed by gas inflow and are connected to black hole growth, providing a new understanding of nuclear disk formation and evolution.

Findings

Simulations reproduce observed properties of M31's nuclear disk.

Lopsided stellar disks are relics of black hole accretion episodes.

Disks influence gas inflow and black hole fueling mechanisms.

Abstract

The physics of angular momentum transport from galactic scales (~10-100 pc) to much smaller radii is one of the oustanding problems in our understanding of the formation and evolution of super-massive black holes (BHs). Seemingly unrelated observations have discovered that there is a lopsided stellar disk 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. Multi-scale 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 disk that propagates inwards from larger radii. The lopsided stellar disk exerts a strong torque on the remaining gas, driving inflows that fuel the growth of the BH and produce quasar-level luminosities. The same disk 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 disk is long lived and retains the eccentric pattern. Simulations that yield quasar-level accretion rates produce relic stellar disks with kinematics, eccentric patterns, precession rates, and surface density profiles in reasonable agreement with observations of M31. The observed properties of nuclear stellar disks can thus be used to constrain the formation history of super-massive BHs.