Formation of Star Clusters and Supermassive Black Holes in Early Universe Galaxies: Insights from a High-Resolution Simulation

Core Concepts

High-resolution simulations reveal that the fragmentation of gas-rich disks in early universe galaxies can lead to the formation of massive, compact star clusters, which may further contribute to the rapid assembly of overmassive supermassive black holes.

Abstract

This research paper investigates the formation of star clusters and supermassive black holes (SMBHs) in the early universe (z > 7) using a high-resolution cosmological hydrodynamical simulation called MassiveBlackPS.

Bibliographic Information: Mayer, L., van Donkelaar, F., Messa, M., Capelo, P. R., & Adamo, A. (2024). In-situ formation of star clusters at z > 7 via galactic disk fragmentation; shedding light on ultra-compact clusters and overmassive black holes seen by JWST. Preprint submitted to The Astrophysical Journal Letters.

Research Objective: The study aims to understand the formation of massive star clusters observed by the James Webb Space Telescope (JWST) in high-redshift galaxies and their potential role in the rapid growth of SMBHs.

Methodology: The researchers utilize the MassiveBlackPS simulation, which employs smoothed-particle hydrodynamics (SPH) and incorporates sub-grid physics models for star formation and supernovae feedback. The simulation focuses on a highly overdense region, mimicking the environment where early galaxies and SMBHs formed.

Key Findings:

• The simulation shows that gas-rich galaxies in the early universe rapidly form massive, compact star clusters through the fragmentation of their disks due to gravitational instability.
• These star clusters have properties (mass, size, and stellar densities) strikingly similar to those observed by JWST in lensed galaxies at z = 7-10.
• The high stellar densities within these clusters could lead to the formation of intermediate-mass black holes (IMBHs), which would then sink to the galactic center and merge, rapidly building up a SMBH.
• The simulations suggest two possible pathways for SMBH growth: (1) direct accretion of star clusters onto a pre-existing SMBH seed in massive galaxies, and (2) merging of IMBHs formed within star clusters in lower-mass galaxies.

Main Conclusions:

• Disk fragmentation is a common mode of star cluster formation in overdense regions of the early universe.
• The formation of massive, compact star clusters can contribute significantly to the rapid growth of SMBHs, potentially explaining the presence of overmassive SMBHs observed in the early universe.

Significance: This research provides a compelling explanation for the formation of massive star clusters and their role in the early growth of SMBHs, offering valuable insights into the processes that shaped the early universe.

Limitations and Future Research: The simulation only covers a short period after the galaxy merger, and the impact of radiative feedback from massive stars on cluster evolution is not fully considered. Future studies should incorporate these aspects and explore the long-term evolution of star clusters and their interaction with SMBHs.

Stats

The simulated star clusters have masses in the range of 10^5–10^8 solar masses and typical half-mass radii of a few parsecs.
The clusters reach characteristic densities above 10^5 solar masses per cubic parsec.
The fractional mass contribution of clusters to the total stellar mass of the galaxies is in the range of 20–40%.
The dynamical friction timescale for the clusters to migrate to the galactic center is estimated to be less than 10^8 years.
The inward mass flux due to cluster migration is estimated to be on the order of 0.1 solar masses per year.

Quotes

Key Insights Distilled From

In-situ formation of star clusters at z > 7 via galactic disk fragmentation; shedding light on ultra-compact clusters and overmassive black holes seen by JWST

by Lucio Mayer,... at arxiv.org 11-04-2024

https://arxiv.org/pdf/2411.00670.pdf

Deeper Inquiries

How would the inclusion of radiative feedback from massive stars in the simulation affect the evolution and final properties of the star clusters?

Including radiative feedback from massive stars would significantly impact the simulation by heating and ionizing the gas within the star clusters. This could alter the clusters' evolution and final properties in several ways:

Reduced Gas Fractions: Radiative feedback could drive gas outflows from the clusters, lowering their gas fractions compared to the current simulation results. This effect would be particularly pronounced in less massive clusters with lower escape velocities. Observational confirmation of the presence or absence of cold gas in high-redshift clusters, potentially through ALMA observations, would be crucial to validate the simulated gas fractions.

Suppression of Star Formation: By heating and expelling gas, radiative feedback could quench star formation within the clusters, leading to lower final stellar masses. The efficiency of this suppression would depend on the intensity of the feedback and the ability of the cluster's gravitational potential to retain the heated gas.

Morphological Changes: The expulsion of gas due to radiative feedback could also affect the morphology of the clusters. They might expand, become less dense, or even dissolve entirely if the feedback is strong enough to overcome their gravitational binding energy.

Impact on IMBH Formation: The formation of intermediate-mass black holes (IMBHs) within the clusters could be influenced by radiative feedback. On the one hand, the reduced gas density might hinder the rapid growth of very massive stars and subsequent IMBH formation. On the other hand, if feedback triggers further star formation in the cluster's outer regions, it could provide more material for IMBH growth through stellar mergers.
In summary, incorporating radiative feedback from massive stars is crucial for a more realistic simulation of star cluster evolution. It would likely lead to lower gas fractions, potentially suppress star formation, and influence the morphology and IMBH formation within these clusters.

Could alternative mechanisms, such as the accretion of gas from the surrounding medium, play a significant role in the growth of SMBHs in these early galaxies?

While the paper focuses on the "IMBH rain" scenario, where SMBHs grow through the merger of star clusters and their embedded IMBHs, alternative mechanisms could indeed contribute to SMBH growth in these early galaxies. Accretion of gas from the surrounding medium is a prominent example:

Direct Gas Accretion: The dense gas disks in these high-redshift galaxies provide a reservoir of material for SMBH growth through direct accretion. This process could be particularly efficient if the gas can lose angular momentum and flow towards the galactic center, potentially fueled by galaxy mergers or disk instabilities.

Supplying the Central SMBH: Even in the presence of the "IMBH rain" scenario, a central SMBH seed might already exist, potentially formed through direct collapse. The inward migration of gas driven by dynamical processes could sustain the growth of this central SMBH alongside the contribution from merging IMBHs.

Co-existence of Mechanisms: It's plausible that both the "IMBH rain" and direct gas accretion contribute to SMBH growth, with their relative importance varying depending on factors like the galaxy's mass, merger history, and gas content.

Observational Constraints: Distinguishing between these scenarios observationally is challenging but crucial. Studying the relationship between SMBH mass, host galaxy properties, and the surrounding gas content in high-redshift galaxies can provide valuable insights into the dominant growth mechanisms.
In conclusion, while the "IMBH rain" scenario offers a compelling explanation for the rapid growth of SMBHs in early galaxies, direct gas accretion from the surrounding medium could play a significant or even complementary role. Future observations and simulations exploring the interplay of these mechanisms are essential to fully understand SMBH growth in the early Universe.

What are the implications of this research for our understanding of the co-evolution of galaxies and their central SMBHs over cosmic time?

This research, highlighting the in-situ formation of massive star clusters and their potential role in SMBH growth, has profound implications for our understanding of the co-evolution of galaxies and their central SMBHs:

Early SMBH Formation: The "IMBH rain" scenario provides a mechanism for the rapid formation of relatively massive SMBHs (around 10^7 solar masses) in early galaxies, even those less massive than those typically associated with bright quasars. This challenges the traditional view that SMBHs form exclusively through the slow growth of light seeds and suggests a more diverse and potentially faster pathway.

Overmassive SMBHs: The scenario naturally explains the recent observations of "overmassive" SMBHs in high-redshift galaxies, where the SMBH mass significantly exceeds what would be expected based on local scaling relations with host galaxy properties. This suggests that early SMBH growth might be decoupled from the overall growth of their host galaxies, at least in certain mass ranges and environments.

Environmental Dependence: The study emphasizes the potential role of environment in shaping SMBH growth. The formation of massive, compact star clusters, and consequently the efficiency of the "IMBH rain" scenario, appears to be favored in highly biased, overdense regions where galaxies grow rapidly. This suggests that SMBH evolution might be intrinsically linked to the large-scale structure of the Universe.

Diverse Growth Channels: The research highlights the possibility of multiple, co-existing channels for SMBH growth, including the "IMBH rain" scenario, direct gas accretion, and potentially others. The relative importance of these channels might vary across cosmic time and depend on factors like galaxy mass, merger history, and gas content.

Feedback and Regulation: The formation and evolution of massive star clusters, as well as the subsequent growth of SMBHs, are expected to have significant feedback effects on the host galaxy. This feedback, in the form of stellar winds, supernova explosions, and AGN activity, can regulate star formation, drive gas outflows, and influence the overall evolution of the galaxy.
In conclusion, this research provides compelling evidence for a new pathway for SMBH formation and growth in the early Universe, highlighting the importance of massive star clusters and their potential to build up substantial black holes rapidly. It emphasizes the complex interplay between SMBHs, their host galaxies, and the surrounding environment, paving the way for a more nuanced understanding of their co-evolution over cosmic time.