ACORN I. Massive Black Hole Seeding and Tidal Disruption Events from Star Clusters in Cosmological Simulations

Abstract: JWST has revealed massive star clusters at $z\gtrsim 6$, which provide promising environments for massive black hole (MBH) seed formation. We introduce a new MBH seeding model for cosmological simulations, physically motivated by the runaway collisions in star clusters. Subgrid star clusters are generated according to the local ISM and evolved through stellar evolution and two-body relaxation. The masses of MBH seeds formed through runaway collisions are determined by the properties of their host clusters, including the effects of metallicity-dependent stellar winds. We further follow tidal disruption events (TDEs) with the evolving clusters, which can be enhanced in MBH binaries, and account for gravitational-wave (GW) recoil following MBH mergers. We implement this model in a constrained simulation of a 5$σ$ overdense region, where seeds of $10<sup>{3}-10<sup>{5.6}\,M_\odot$ form in clusters with $$10^{4}\leq m_{\rm SC}\leq 10<sup>{8}\,M_\odot$$. Because both the seed abundance and mass are directly connected to the local environment, we have a more physical $M_{\rm BH}!-!M_{\rm galaxy}$ scaling for low-mass galaxies ($M_{\rm galaxy}\sim10<sup>{6-9}\,M_\odot$), a regime poorly described by previous seeding models. Multiple MBHs are naturally allowed to form in-situ within individual halos, producing a spatially clustered population. This increases the MBH merger rate by $\sim100$ relative to other seeding models, significantly enhancing the expected LISA event rate. TDEs can dominate the growth of $10<sup>{3-4}\,M_\odot$ MBHs and produce luminous flares of $$L_{\rm peak}\sim 10^{{42-43}\,{\rm} erg\,s<sup>{-1}}$$, increasing the observability of low-mass seeds. This seeding prescription accelerates early MBH growth at $z\gtrsim 8$, while subsequent self-regulated gas accretion drives the central MBH toward similar masses $\sim10^{8.6}\, M_\odot$ by $z=6$ across different seeding models.