Thousands of black holes form disks in the centers of galaxies

The region around a galaxy's supermassive black hole may harbor thousands of stellar-sized black holes, orbiting in a disk.
By | Published: September 13, 2018 | Last updated on May 18, 2023
In this artistic visualization, a supermassive black hole at a galaxy’s center shoots out radiation and high-speed winds. According to a new study, supermassive black holes at a galaxy’s center are surrounded by a disk of black holes and massive stars.
At the center of most galaxies lie supermassive black holes. Their exceptional gravity pulls in thousands of stars and stellar mass black holes, or black holes formed when a massive star collapses due to gravity.

By simulating how objects interact near the supermassive black holes in the center of galaxies, astrophysicists from Eötvös University in Hungary have shown, in a new study, that these black holes form a thick disk around a galaxy’s supermassive black hole.

“Previously it was thought that the orbits of both light and massive stellar objects are distributed [uniformly] around the supermassive black hole,” Ákos Szölgyén, a researcher at Eötvös University who led the study, said in a statement, “we now understand that massive stars and black holes typically segregate into a disk.”

An artist’s illustration of the black hole Cygnus X-1. New research shows that thick disks of black holes and massive stars likely form at the center of all galaxies, surrounding supermassive black holes.
Swarm of Black Holes

In their simulation, Szölgyén and his Ph.D. advisor, Bence Kocsis, incorporated something called vector resonant relaxation. It’s an effect that gravity has on objects orbiting a supermassive black hole. This effect grows over millions of years, making the orbital planes of these objects turn.

Kocsis compared the effect and the behavior of the objects to the movement of bees, “Unlike a swarm of bees around a beehive, stars fly around in the galactic center in a more ordered way: along precessing elliptical trajectories, each confined to a plane, respectively,” he said in the statement. Kocsis continued, describing how the objects shift their orbits slowly over millions of years.

This effect helped the astronomers see that while black holes orbit in a disk, less massive objects like stars form a more spherical distribution, Kocsis added in an email.

Stars usually form in one of two ways at the centers of galaxies. Gas can condense into stars around the supermassive black hole. Or, alternatively, groups of stars called globular clusters can spin into the galaxy’s center, where they’re ripped into the building blocks of new stars by the supermassive black hole. “In both cases, we find a disk of black holes,” Kocsis noted.

That means these black hole disks probably form in all galaxies.

Black Hole Disks and Gravitational Waves

According to Kocsis, this study could also help scientists better understand gravitational waves. As scientists have detected gravitational waves using LIGO and VIRGO, they’ve been surprised to see the rate of black hole mergers is much higher than they expected. “The big question, known as the ‘final AU problem’,” Kocsis explained, is how black holes might get to an AU (or astronomical unit, roughly the distance from the Earth to the sun) that drives them to merge.

According to Kocsis, better understanding black hole disks could help answer this question because these dense environments “may lead to mergers more often.”

This study was published in the journal Physical Review Letters.

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