Supermassive Black Hole Binary Environments: Exploring Pre-Merger Minidisk Accretion and Post-Merger Stellar Orbital Dynamics

Abstract

Supermassive black hole binaries (SMBHBs) are expected to form as a result of galaxymergers. In this dissertation, I present work that investigates the dynamics around SMBHBs investigating accretion in the final stages of their inspiral using numerical general relativistic magnetohydrodynamic (GRMHD) simulations, and the stellar dynamics of the surrounding star cluster after the black holes have merged and experience a gravitational wave (GW) recoil kick using analytic and N-body methods. For investigating the inspiral phase, I perform GRMHD simulations of accreting, equal-mass SMBHBs in the final stages of their inspiral, focusing on the structure and evolution of minidisks around each black hole, and the effect of the existence of minidisks on the accretion flow. We find that minidisks form only when the Hill sphere radius is significantly larger than the effective innermost stable circular orbit (ISCO) around each black hole. We furthermore find that the strength of the periodic modulation in the accretion rate is affected by the existence of persistent minidisks, with systems that are able to maintain minidisks exhibiting much weaker modulation in the accretion rate. For the post-merger phase I investigate the effect of a gravitational wave recoil kick on the surrounding stellar population of the SMBHB merger remnant using both analytic and N-body methods. We find that an initially circular disk of stars will be distorted into an eccentric, apsidally-aligned disk. We characterize the exact regions in the disk where stars will become unbound, flipped to be retrograde, have anti-alignment, and be on smaller or larger semi-major axis orbits relative to their pre-kick orbits. We also characterize the the post-kick orbital eccentricities as a function of position in the disk, and make predictions for the average eccentricity and apsidal alignment of the disk, as well the time that the first star would enter the tidal disruption radius of the SMBH. We also use N-body simulations to evolve segments of disks with different retrograde fractions and show that disks with significant counter-rotation are more stable (i.e. apsidal alignment is most pronounced and long lasting), more eccentric, and have the highest rates of stars entering the SMBH’s tidal disruption radius.