Stellar Collisions from Self-consistent Stellar Dynamics around Growing Supermassive Black Holes

Abstract The centers of galaxies harbor the densest stellar environments, where a massive black hole (MBH) accelerates stars to such high velocities that direct collisions can result in high-energetic phenomena, such as gravitational wave sources, kilonovae, and supernova-like transients. These collisions can reshape the cluster’s density profile and release gas that can be subsequently accreted by the MBH. However, the evolving rates of such phenomena from self-consistent dynamics around mass-growing MBHs remain largely unexplored. In this work, we simulate nuclear star clusters (NSCs) across a range of masses and density profiles by employing the GNC Monte Carlo code, which self-consistently models stellar dynamics and the subsequent accretion of released gas. We find that stellar collisions flatten the density cusp in the innermost regions ( r ≲ 10 −3 −10 −2 pc) within ∼0.1−1 Gyr. While high initial collision rates in steep cusps quickly decline due to stellar depletion, the interplay between collisions and MBH growth is important only in massive NSCs ( M ⋆ ∼ 10 9 M ⊙ ). As MBH grows, increased stellar velocities shift the balance toward destructive collisions, the relative velocities of which can be ≳2500 km s −1 . Consequently, present-day destructive collision rates in massive clusters remain high (10 −4 ∼ 10 −3 yr −1 ), whereas they are smaller in Milky Way–like NSCs or negligible in smaller NSCs. Our results highlight a crucial synergy between stellar dynamics and MBH growth, identifying massive galaxies as prime targets for observing transients from destructive stellar collisions.