Wind-fed Supermassive Black Hole Accretion by the Nuclear Star Cluster: The Case of M31*

Abstract

The central supermassive black hole (SMBH) of the Andromeda galaxy, known as M31*, exhibits dim electromagnetic emission and is inferred to have an extremely low accretion rate for its remarkable mass (∼108M⊙). In this work, we use three-dimensional hydrodynamical simulations to explore a previously untested scenario, in which M31* is fed by the collective stellar mass loss from its surrounding nuclear star cluster, manifested as a famous eccentric disk of predominantly old stellar populations. The stellar mass loss is assumed to be dominated by the slow and cold winds from 100 asymptotic giant-branch stars, which follow well-constrained Keplerian orbits around M31* and together provide a mass injection rate of ∼4 × 10−5M⊙ yr−1. The simulations achieve a quasi-steady state on a megayear timescale, at which point a quasi-Keplerian, cool (T ∼ 103–104 K) gas disk extending several parsecs is established. This disk is continuously supplied by the stellar winds and itself feeds the central SMBH. At the end of the simulations at 2 Myr, an accretion rate of ∼2 × 10−5M⊙ yr−1 is found but could vary by a factor of a few depending on whether the subdominant gravity of the NSC or a moderate global inflow is included. The predicted X-ray luminosity of ∼1036 erg s−1, dominated by the hot (T ∼ 107–108 K) plasma within 0.2 pc of the SMBH, is consistent with Chandra observations. We conclude that the feeding mechanism of M31* is successfully identified, which has important implications for the working of dormant SMBHs prevalent in the local Universe.