Globular Clusters as Cosmic Black Hole Factories
Angeloni et al. investigate whether globular clusters (GCs), dense, ancient groups of stars, can act as “nurseries” for dynamically-formed binary black holes (BBHs), especially the most massive systems detected through gravitational waves. Motivated by puzzling observations from the LIGO–Virgo–KAGRA (LVK) Collaboration, which include black holes heavier than expected from standard stellar evolution, the authors focus on hierarchical mergers: repeated black hole mergers occurring in dense environments that can build up very massive black holes over time. Their goal is to connect black hole mergers directly to galaxy and cluster formation across cosmic history using a single, self-consistent framework .
Simulating a Milky Way–Like Universe
The study begins by describing the galaxy formation model GAMESH, which simulates a Local Group–like region containing a Milky Way–analog galaxy. This model follows dark matter, gas, star formation, and chemical enrichment from very early times (redshift (z=20)) to the present day. Within this evolving environment, the authors identify where and when globular clusters can form, based on conditions such as high gas surface density and star formation rate. Because the simulated region is slightly overdense compared to the average Universe, the authors emphasize that many of their results should be interpreted as upper limits when compared to cosmological averages.
Forming and Destroying Globular Clusters
Next, Angeloni et al. explain how they couple globular cluster formation to cluster population synthesis (CPS) codes, which are designed to efficiently model the internal dynamics of star clusters and their black hole populations. Newly formed clusters are assigned masses, sizes, and metallicities tied to their host galaxies, and then evolved forward in time. Crucially, the authors include realistic destruction mechanisms, such as tidal disruption by giant molecular clouds (the “cruel cradle effect”) and galaxy mergers, to avoid overproducing surviving clusters. This allows the simulated Milky Way globular cluster population to be meaningfully compared with observations.
Modeling Black Holes in Dense Star Clusters
To model black hole formation inside clusters, the authors use two independent CPS codes, RAPSTER and FASTCLUSTER. Both track processes like three-body encounters, black hole binary formation, gravitational-wave captures, and hierarchical merger chains, but they differ in their internal assumptions and calibration. By comparing results from both codes under matched initial conditions, the authors highlight how sensitive predictions for black hole masses and merger rates are to the details of cluster physics, an important caveat for interpreting gravitational-wave data.
Matching the Milky Way’s Globular Clusters
The results show that the simulated globular cluster population broadly reproduces the observed age and mass distribution of Milky Way clusters, although the metallicities tend to be higher than observed, likely due to simplified chemical enrichment assumptions. About 30% of present-day halo globular clusters are found to originate from satellite galaxies later accreted by the Milky Way. Importantly, the densest and most massive clusters are identified as the key sites where massive binary black holes (MBBHs), with component masses above 50 solar masses, can form through repeated mergers.
Building Massive and Intermediate-Mass Black Holes
When examining the black hole population itself, the authors find that hierarchical mergers in globular clusters naturally produce black holes in and above the pair-instability mass gap, a long-standing theoretical challenge. Both CPS codes predict a distinct high-mass peak in the primary black hole mass distribution, a feature not easily explained by isolated binary evolution alone. RAPSTER additionally predicts the formation of intermediate-mass black holes (IMBHs), some of which may become “wandering” black holes in galactic disks or halos after their host clusters are destroyed.
Looking Back to the Early Universe
Finally, the paper explores the cosmic evolution of BBH birth and merger rates. Angeloni et al. show that both rates increase strongly with redshift, peaking around (z ~ 3-5), close to the epoch of globular cluster formation. Most mergers occur too early and too far away to be detected by current observatories, but this trend has major implications for future detectors such as LISA, the Einstein Telescope, and Cosmic Explorer, which will be able to probe the high-redshift Universe. Overall, the study demonstrates that globular clusters are likely a major, and testable, channel for producing the most massive black holes observed through gravitational waves.
Source: Angeloni
