Star Cluster Collisions May Seed Intermediate Mass Black Holes

Simulations of massive star cluster formation reveal intermediate-mass black holes (IMBHs) form via stellar collisions and mergers within the first 10 million years. IMBH mass correlates with initial cluster mass, density, and velocity dispersion, potentially explaining observed dense star cluster populations with IMBH masses exceeding solar masses.

The formation of supermassive black holes, entities at the heart of most galaxies, remains a significant challenge in astrophysics. Current models propose these behemoths originate from smaller ‘seed’ black holes, but the precise mechanisms driving their initial growth are debated. Recent research focuses on the role of dense star clusters as potential nurseries for these seeds, where stellar collisions and mergers can rapidly build up mass. A collaborative team, comprising Antti Rantala, Natalia Lahén and Thorsten Naab from the Max-Planck-Institut für Astrophysik, alongside Gastón J. Escobar from both the Instituto de Astrofísica de Canarias and the Universidad de La Laguna, and Giuliano Iorio from the Universitat de Barcelona, have investigated this process through detailed computational modelling. Their work, detailed in the paper ‘FROST-CLUSTERS — II. Massive stars, binaries and triples boost supermassive black hole seed formation in assembling star clusters’, demonstrates that the presence of multiple star systems – binaries and triples – and a high upper limit on initial stellar mass significantly enhances the formation of intermediate-mass black holes within these clusters, potentially providing the seeds for larger supermassive counterparts.

Stellar Nurseries Forge Intermediate-Mass Black Holes: Simulations Reveal Formation Pathways in Dense Star Clusters

Massive star clusters represent dynamic environments where stars form, evolve, and interact gravitationally, creating conditions conducive to black hole formation. Recent advances in computational astrophysics enable scientists to simulate these complex systems with unprecedented detail, revealing crucial insights into the formation of intermediate-mass black holes (IMBHs). These simulations demonstrate how stellar interactions within dense star clusters drive IMBH formation, establishing a connection between stellar dynamics and the growth of these elusive objects. Researchers utilise sophisticated models incorporating stellar evolution, gravitational dynamics, and radiative transfer to explore the conditions necessary for IMBH formation and predict their prevalence in the universe.

Simulations employing post-Newtonian dynamics – an approximation of general relativity used for weak gravitational fields – and detailed stellar evolution models demonstrate that massive star clusters assemble through a hierarchical process, initiating a cascade of interactions that ultimately lead to IMBH formation. These models, utilising the \bifrost{} code coupled with the \sevn{} stellar evolution module, reveal the significant influence of initial stellar populations on cluster development, highlighting the importance of binary and multiple star systems. Specifically, the presence of initial binary and triple star systems demonstrably affects the cluster’s structure, kinematics, and IMBH formation, establishing a clear link between stellar multiplicity and black hole growth.

These simulations indicate that IMBH formation occurs through multiple channels within these dense stellar environments, creating a diverse population of black holes with varying masses and origins. Stellar collisions, tidal disruption events (TDEs), and black hole mergers all contribute to IMBH growth, showcasing the dynamic nature of these systems. Stellar collisions, particularly frequent in the dense core of star clusters, can directly produce massive stars that subsequently collapse into black holes, while TDEs – where a star is torn apart by a black hole’s gravity – provide a source of material for black hole accretion. Black hole mergers, driven by dynamical interactions within the cluster, further contribute to IMBH growth, creating even more massive objects.

Notably, the number and mass of formed IMBHs – reaching up to $10^5$ solar masses – increase with higher stellar multiplicity and a greater initial single star mass limit, demonstrating the importance of these parameters in driving black hole formation. Up to ten IMBHs can form within the first 10 million years of cluster evolution, showcasing the efficiency of this process in dense stellar environments.

Tidal disruption events (TDEs) exhibit peak rates shortly after IMBH formation, challenging previous assumptions about the upper mass limit of stellar-mass black holes. The existence of such massive IMBHs has significant implications for our understanding of black hole formation and evolution, as well as the formation of supermassive black holes at the centers of galaxies.

Crucially, initial stellar multiplicity and a high mass limit also promote mergers between IMBHs, leading to the formation of even more massive black holes and potentially contributing to the growth of supermassive black holes at the centers of galaxies. These mergers are particularly efficient in dense star clusters where the black hole density is high and dynamical interactions are frequent.

Future research will focus on incorporating more realistic physics into these simulations, including the effects of gas dynamics, stellar rotation, and magnetic fields. These improvements will allow for a more accurate representation of the complex processes occurring within dense star clusters and provide a more complete picture of IMBH formation and evolution. Furthermore, ongoing and future gravitational wave observatories will provide a wealth of data that can be used to test the predictions of these simulations and refine our understanding of the black hole population in the universe.

By combining the power of computational astrophysics with observational data, scientists are making significant progress in unraveling the mysteries of IMBH formation and evolution. These efforts will not only deepen our understanding of black holes but also shed light on the formation and evolution of galaxies and the universe as a whole. The dynamic environments of dense star clusters continue to reveal themselves as crucial sites for the birth and growth of these enigmatic objects, offering a glimpse into the most extreme environments in the cosmos.