In a recent study, Lucio Mayer and his team explore how star clusters and black holes formed in galaxies during the early universe, specifically at redshifts greater than 7. Using advanced simulations, they demonstrate that dense, gas-rich disks in primordial galaxies could undergo fragmentation, leading to the rapid formation of ultra-compact star clusters. These findings shed light on the dense star clusters and overmassive black holes recently observed by the James Webb Space Telescope (JWST).

Background and Motivation

The discovery of dense star clusters and unexpectedly large black holes in galaxies at high redshifts (z > 7) challenges current understandings of galaxy and black hole formation. The JWST has observed these clusters in very distant galaxies, sparking questions about their origins and the processes driving early galaxy evolution. Mayer’s team uses simulations to explore how these structures might have formed in galaxies embedded in highly dense regions of the universe, seeking to uncover whether disk fragmentation could explain the compact clusters and massive black holes.

Simulation Approach

The researchers ran high-resolution simulations within a cosmological model to recreate early galaxy environments. Using the MassiveBlackPS simulation, they focused on a massive galaxy and its smaller companions, setting conditions to replicate a dense region with high gas content. The simulated galaxy disk was unstable under its own gravity, making it prone to fragmentation. This setup allowed the team to observe how gas clumps formed within these disks and quickly evolved into compact star clusters.

Formation of Compact Star Clusters

Mayer’s team found that the simulated galaxies produced dense, gravitationally bound gas clumps, which quickly converted gas into stars, forming compact star clusters. These clusters, with sizes around a few parsecs, reached stellar densities exceeding 100,000 solar masses per cubic parsec, similar to clusters observed by the JWST. In the smallest simulated galaxy, these clusters formed in a highly compact arrangement, with masses ranging from 100,000 to several million solar masses, mirroring the properties of the dense star clusters in the Cosmic Gems arc observed at z = 10.2.

Growth of Black Holes

The study suggests that the extreme densities within these star clusters could facilitate the formation of intermediate-mass black holes (IMBHs). These IMBHs, once formed, would migrate toward the galactic center and eventually merge, assembling a supermassive black hole (SMBH) in the process. Mayer’s team proposes that this “IMBH rain” scenario could explain the overmassive black holes JWST has observed. In this process, both gas inflow and cluster mergers contribute to the central black hole’s growth, allowing it to reach a mass exceeding 10% of its host galaxy’s stellar mass.

Implications for Galaxy Evolution

The study’s results suggest that high-redshift galaxies could produce compact clusters and IMBHs through a process of disk fragmentation, especially in dense regions where gas remains stable enough for gravitational collapse. This mechanism could be prevalent in early, gas-rich galaxies, leading to widespread formation of overmassive SMBHs in smaller galaxies, as observed by JWST.

Conclusions and Future Directions

Mayer’s simulations offer a compelling explanation for the presence of ultra-compact star clusters and overmassive black holes in the early universe, attributing their formation to gravitational instabilities in dense, gas-rich disks. Future research might focus on observational confirmation through ALMA and other telescopes to detect gas within these clusters, supporting or challenging the disk fragmentation model. Mayer’s findings underscore the role of dense environments in shaping galaxy evolution and contribute to a broader understanding of how SMBHs formed during the universe’s infancy.

Source: Mayer