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, that 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 ($r \lesssim 10^{-3}-10^{-2}$ pc) within $\sim 0.1-1$ 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 ($M_\star \sim 10^9 M_\odot$). As MBH grows, increased stellar velocities shift the balance toward destructive collisions of which relative velocities can be $\gtrsim 2500 {\rm \,km\,s^{-1}}$. Consequently, present-day destructive collision rates in massive clusters remain high ($10^{-4}\sim 10^{-3}{\rm yr}^{-1}$), 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.