A new cosmological simulation suggests that dense star clusters in the early universe could have formed seeds for supermassive black holes through runaway mergers. This work addresses how these massive objects grew rapidly after the universe's birth and may explain mysterious 'little red dots' observed by the James Webb Space Telescope. Led by Fred Garcia at Columbia University, the study recreates the first 700 million years of cosmic history.
Supermassive black holes reside at the cores of nearly all of the 2 trillion galaxies in the universe. These monsters formed soon after the universe's birth and grew to millions or billions of times the sun's mass in less than a billion years, puzzling astronomers.
The mystery deepened in 2022 when the James Webb Space Telescope (JWST) detected 'little red dots' in images of the distant universe. These appear around 600 million years after the universe's birth and faded within the next billion years, with many scientists now viewing them as growing supermassive black holes appearing earlier than expected.
Fred Garcia, a graduate student at Columbia University, led a study published in The Open Journal of Astrophysics (DOI: 10.33232/001c.145064). It supports the idea that dense star clusters formed intermediate-mass black hole seeds. 'Right now, there are three main ideas for how supermassive black hole seeds form: direct collapse from gas clouds, remnants of the first stars, and dense star clusters,' Garcia said. 'Our work really supports the last case.'
The simulation focused on a dwarf galaxy's formation in the first 700 million years, revealing two bursts of star formation in cold gas clouds within a dark matter halo. Clusters migrated to the center, merging into a nuclear star cluster shining like a million suns. Some clouds converted up to 80 percent of gas into stars, far exceeding the 2 percent in modern galaxies.
Stars in the dense cluster exploded, leaving black holes that sank to the core and merged repeatedly. 'All the stellar-mass black holes that migrate to the center... eventually form their own incredibly dense, gravitationally bound cluster,' said Matías Liempi Gonzalez of Sapienza University of Rome, who was not involved. This runaway process could create supermassive black hole embryos.
Priyamvada Natarajan of Yale University, also uninvolved, noted the findings align with her 2014 theoretical work on black hole growth in such clusters. The simulation may explain JWST sightings of star clusters at 460 and 600 million years post-big bang, with future observations from JWST and the LISA mission potentially confirming the link.