Spins of Intermediate Mass Black Holes in Galactic Nuclei from Black Hole-Star Collisions

In the inner 0.1 parsec of nuclear star clusters, black holes (BHs) frequently collide with stars due to the high density and velocity dispersion in these regions. During such collisions, BHs accrete mass from the stars. With repeated collisions, BHs can accumulate significant mass, potentially growing beyond 100 M⊙. This is important because intermediate mass black holes (IMBHs), which exceed the mass of BHs formed solely through stellar death, are likely formed through collisions or other processes. BHs are characterized by their mass and spin. Our study focuses on how spin evolves during these collisions. We track BH spin change in a simulation over numerous collisions, and assume the accretion disk is either prograde or retrograde relative to initial spin. Retrograde orbits carry more angular momentum because the innermost stable circular orbit (ISCO) is further from the BH. As a result, over many collisions, we find the final BH spin is near zero. This mechanism therefore represents a channel to produce low spinning massive BHs.