Hubble uncovers rare black hole in Omega Centauri

An international team of astronomers has created an enormous catalogue of the motions of stars in Omega Centauri, measuring the velocities for 1.4 million stars by studying over 500 images from the Hubble Space Telescope spanning two decades. Most of these observations were intended for instrument calibration rather than scientific use, but they turned out to be an ideal database for the team’s research efforts.

The extensive catalogue, which is the largest catalogue of motions for any star cluster to date, will be made openly available. “We discovered seven stars that should not be there,” explained Maximilian Häberle of the Max Planck Institute for Astronomy in Germany, who led this investigation. “They are moving so fast that they should 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 centre. The only object that can be so massive is a black hole, with a mass at least 8200 times that of our Sun.”

The discovery of these seven fast-moving stars provides compelling evidence for the presence of an intermediate-mass black hole (IMBH) in Omega Centauri. Several studies have previously suggested the existence of such a massive object in the cluster’s core, but other studies proposed alternative explanations, such as a central cluster of stellar-mass black holes. However, the lack of observed stars moving at velocities high enough to escape the cluster’s gravitational pull made an IMBH seem less likely.

The team’s findings change this perspective. “This discovery is the most direct evidence so far of an IMBH in Omega Centauri,” said team lead Nadine Neumayer of the Max Planck Institute for Astronomy, who initiated the study. “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 IMBH in our cosmic neighborhood.”

If confirmed, at a distance of 17,000 light-years, this IMBH candidate would reside closer to Earth than the supermassive black hole at the center of the Milky Way, which is 26,000 light-years away. Besides the galactic center, it would also be the only known case of a number of stars closely bound to a massive black hole.

While the black hole is believed to measure at least 8,200 solar masses, its exact mass and precise position are not fully known. The science team hopes to characterize the black hole further by studying the orbits of the fast-moving stars, which requires additional measurements of their line-of-sight velocities. The team has been granted time with the James Webb Space Telescope to carry out these observations and has other pending proposals to use other observatories.