Black Hole Growth Linked to Early Universe Galaxy Evolution.

Research reveals the black hole-stellar mass relation evolves in the early Universe, decreasing from 0.1 to 0.01 with redshift. Simulations, utilising the black hole-dynamical mass relation, accurately reproduce observed distributions, including overmassive black holes in low-mass galaxies, influenced by decreasing dark matter and gas fractions.

The early Universe presents persistent challenges to cosmological models, particularly concerning the unexpectedly large black holes observed at high redshifts. Current theories struggle to account for the rapid accretion needed to form these ‘overmassive’ black holes within the limited timeframe following the Big Bang. New research suggests a plausible explanation rooted in the characteristics of the galaxies hosting these black holes. A team led by Sandro Tacchella, William McClymont, and Roberto Maiolino, all from the Kavli Institute for Cosmology at the University of Cambridge, alongside collaborators from institutions including the Leibniz-Institut für Astrophysik and York University, present evidence that gas-rich, dark matter-dominated galaxies naturally facilitate the growth of these overmassive black holes. Their findings, detailed in a paper titled ‘Overmassive black holes in the early Universe can be explained by gas-rich, dark matter-dominated galaxies’, utilise high-resolution simulations and observational data from the JADES survey to demonstrate a consistent relationship between black hole mass and galaxy properties at redshifts exceeding six. The team, which also includes researchers such as Charlotte Simmonds, Mark Vogelsberger, and Xuejian Shen, predicts black hole masses based on a fundamental relationship with stellar mass, successfully reproducing observed distributions and accounting for the prevalence of these unexpectedly large black holes in the early cosmos.

Recent investigations utilising high-resolution cosmological simulations, known as THESAN-ZOOM, reveal a consistent relationship between black hole (BH) mass and stellar mass in the early Universe, while simultaneously demonstrating an evolving connection with increasing redshift. Redshift, a measure of how much the light from an object has been stretched due to the expansion of the Universe, serves as a proxy for the time at which we observe it; higher redshift corresponds to earlier times. The simulations predict black hole masses across a range of simulated galaxies by assuming a fundamental relation, achieving notable agreement with observed distributions without explicitly modelling black hole growth within the simulations. This approach successfully reproduces the presence of overmassive black holes, those exceeding expected mass relative to locally observed scaling relations.

The simulations demonstrate a decline in the ratio of black hole mass to stellar mass with increasing redshift, transitioning from approximately 0.1 at a redshift of 6 to 0.01 at a redshift of 8. This indicates a shift in the relative contributions of black hole and stellar mass to the total mass of a galaxy. This trend correlates with decreasing dark matter and gas fractions within the galaxies, suggesting conditions that favour rapid black hole growth relative to star formation. Consequently, low-mass galaxies at high redshift exhibit smaller ratios and, therefore, host overmassive black holes, challenging conventional expectations regarding the established scaling relations between black holes and their host galaxies.

Researchers validate these findings through the analysis of 48,022 galaxies within the JADES survey at a redshift of 6, utilising derived stellar masses and star-formation rates to infer the black hole-stellar mass relation. The results demonstrate strong agreement between the simulation results and observational data, reinforcing the conclusion that overmassive black holes naturally arise from a fundamental relation operating under the unique conditions of the early Universe. This concordance provides compelling evidence for a specific evolutionary pathway for black holes and galaxies, offering a valuable benchmark for future studies.

The implications of this research extend to understanding the initial stages of galaxy formation, as the prevalence of rapidly growing, overmassive black holes suggests a significant influence on the evolution of their host galaxies. These black holes likely played a crucial role in regulating star formation and shaping the morphology of early galaxies, offering insights into the processes that drove the assembly of the structures observed today. By establishing a clear connection between black hole growth and galaxy evolution, this study provides a framework for interpreting observations of high-redshift galaxies and refining models of black hole-galaxy co-evolution.

Future research will focus on refining the simulation framework to incorporate more detailed physics, such as the effects of active galactic nuclei (AGN) feedback – the energy released by material falling into a black hole – and the role of galaxy mergers in driving black hole growth. Investigating the evolution of black hole spin and its impact on galaxy evolution will also be a key area of focus. By combining high-resolution simulations with observations from next-generation telescopes, researchers aim to unravel the complex interplay between black holes and their host galaxies and gain a deeper understanding of the formation and evolution of the Universe.