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Astronomers Find Intermediate-Mass Black Hole in Milky Way’s Most Massive Globular Cluster

Using more than 500 images from the NASA/ESA Hubble Space Telescope, astronomer have found evidence for a 20,000-solar-mass black hole at the center of Omega Centauri, a globular cluster located in the constellation of Centaurus at a distance of 5,430 parsecs (17,710 light-years) from the Sun.

Omega Centauri is about 10 times as massive as other big globular clusters. Image credit: NASA / ESA / Hubble / Maximilian Häberle, MPIA.

Omega Centauri is about 10 times as massive as other big globular clusters. Image credit: NASA / ESA / Hubble / Maximilian Häberle, MPIA.

Astronomers know that stellar-mass black holes — black holes ranging from 10 times to 100 times the Sun’s mass — are the remnants of dying stars, and that supermassive black holes, more than 1,000,000 times the mass of the Sun, inhabit the centers of most galaxies.

But scattered across the Universe are a few apparent black holes of a more mysterious type.

Ranging from 100 to 10,000 solar masses, these intermediate-mass black holes are so hard to measure that even their existence is sometimes disputed.

Only a few intermediate-mass black hole candidates have been found to date.

Determining their population is an important step towards understanding supermassive black hole formation in the early Universe.

“Omega Centauri is a special case among the globular clusters of the Milky Way,” said Max Planck Institute for Astronomy astronomer Maximilian Häberle and his colleagues.

“Owing to its high mass, complex stellar populations and kinematics, Omega Centauri is widely accepted as the stripped nucleus of an accreted dwarf galaxy.”

“These factors combined with its proximity have made it a prime target for searching for an intermediate-mass black hole.”

Omega Centauri consists of roughly 10 million stars and is about 10 times as massive as other big globular clusters.

In their study, the authors measured the velocities for 1.4 million stars gleaned from the Hubble images of the cluster.

Most of these observations were intended to calibrate Hubble’s instruments rather than for scientific use, but they turned out to be an ideal database for the team’s research efforts.

“We searched for fast-moving stars expected to exist near concentrated masses, like black holes,” said University of Queensland astronomer Holger Baumgardt.

“Identifying these stars was the smoking gun evidence we needed to prove the black hole’s existence, and we did.”

“We discovered seven stars that should not be there,” Dr. Häberle said.

“They are moving so fast that they would escape the cluster and never come back.”

“The most likely explanation is that a very massive object is gravitationally pulling on these stars and keeping them close to the center.”

“The only object that can be so massive is a black hole, with a mass at least 8,200 times that of our Sun.”

“This discovery is the most direct evidence so far of an intermediate-mass black hole in Omega Centauri,” said Dr. Nadine Neumayer, an astronomer at the Max Planck Institute for Astronomy.

“This is exciting because there are only very few other black holes known with a similar mass.”

“The black hole in Omega Centauri may be the best example of an intermediate-mass black hole in our cosmic neighborhood.”

The team’s paper was published in the journal Nature.

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M. Häberle et al. 2024. Fast-moving stars around an intermediate-mass black hole in ω Centauri. Nature 631, 285-288; doi: 10.1038/s41586-024-07511-z

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