Researchers have tackled one of the biggest mysteries in modern astronomy - explaining the origin of unusually large black holes found in some of the earliest galaxies in the Universe.
These 'overmassive black hole galaxies (OBGs)', discovered by the James Webb Space Telescope, appeared far earlier than expected and challenge existing theories of how black holes and galaxies formed and grew.
Scientists from the universities of Portsmouth and the United Arab Emirates have published a paper explaining how supermassive black holes emerged so quickly after the Big Bang, which could reshape our understanding of the earliest stages of galaxy evolution.
Dr Daniel Whalen , from the University of Portsmouth's Institute of Cosmology and Gravitation , said: "OBGs are the new frontier of high-redshift black hole astronomy, probing galaxy formation just 450 million years after the Big Bang. Their black holes are about 100 times more massive relative to their host galaxies than those in the Universe today and until now have defied conventional astrophysical explanation."
The team used one of the most detailed cosmological simulations to date, to track a black hole that formed when the Universe was still in its infancy.
Collapsing from a primordial star 70,000 times the mass of the Sun, the black hole grew to six million solar masses over several hundred million years. Meanwhile, its host galaxy developed hundreds of millions of solar masses in stars and reached properties similar to some of the earliest galaxies observed by the James Webb Space Telescope.
Dr Whalen and colleagues showed that these objects may arise naturally from the birth of 'direct collapse black holes', an early type of massive black hole thought to form when giant clouds of primordial gas create supermassive primordial stars that collapse directly to black holes without exploding.
The simulation reproduced key features of recently discovered galaxies, including unusually high black hole to stellar mass ratios, as well as spectral signatures observed in real data.
The researchers found that two processes may explain these extreme systems:
Early suppression of star formation: The growing black hole initially slowed the formation of new stars as it rapidly grew in its host galaxy.
Powerful stellar explosions: Later generations of the Universe's first stars exploded as supernovae, ejecting heavy elements and reshaping the galaxy's evolution.
Together, these effects allowed black holes to grow disproportionately large compared with their galaxies, producing systems that resemble those observed in the early Universe.
Dr Muhammad Latif, Associate Professor from the United Arab Emirates University, said: "Our findings show that OBGs represent a natural evolutionary phase of direct-collapse black hole host galaxies, and their detection by JWST at such early epochs provides direct evidence for the existence of direct-collapse black holes. To validate our simulations, we compared the spectra of the simulated galaxies with the JWST-observed OBGs GHZ9 and UHZ1, finding that our models remarkably reproduce their observed properties and spectra."
The study is the first to simulate the long-term coevolution of a direct collapse black hole and its host galaxy with high-fidelity physics while also capturing star formation in the Universe's earliest small structures. The results suggest that these puzzling galaxies may not be anomalies after all, but instead a natural outcome of black hole formation in the young cosmos.
The paper is published in the Astrophysical Journal Letters .
