Intermediate-mass black holes (IMBHs) are considered the 'missing link' in black hole evolution. Unlike the well-documented supermassive black holes at the centers of galaxies or the smaller black holes under 100 solar masses, IMBHs are rare and elusive. Their existence prompts questions about their prevalence, formation, and whether they contribute to the growth of supermassive black holes. Are dense star clusters like Omega Centauri their preferred habitat?

Omega Centauri is visible to the naked eye from Earth and is a favorite among southern hemisphere stargazers. Located 17,000 light-years away, just above the Milky Way's plane, it appears nearly as large as the full Moon when viewed from a dark area. Its classification has evolved with advancements in astronomical studies. Initially listed by Ptolemy nearly 2000 years ago as a single star, it was identified as a nebula by Edmond Halley in 1677, and John Herschel classified it as a globular cluster in the 1830s.

Globular clusters contain up to one million old stars bound by gravity and are found both at the edges and centers of many galaxies, including our own. Omega Centauri stands out with its rapid rotation, highly flattened shape, and mass, which is about ten times greater than other large globular clusters, nearly equivalent to a small galaxy.

With around 10 million stars, Omega Centauri's extensive catalogue of stellar motions was created by an international team, measuring velocities for 1.4 million stars using over 500 Hubble images. While most images were initially meant to calibrate Hubble's instruments, they provided an invaluable database for this research. The resulting catalogue is the largest of its kind and is publicly available.

"We discovered seven stars that should not be there," explained Maximilian Haberle 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."

Several studies have suggested the presence of an IMBH in Omega Centauri. However, other studies proposed that the mass could be attributed to a central cluster of stellar-mass black holes, with a lack of fast-moving stars above the necessary escape velocity making an IMBH less likely.

"This discovery is the most direct evidence so far of an IMBH in Omega Centauri," added team lead Nadine Neumayer, also 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 neighbourhood."

If confirmed, the candidate black hole at 17,000 light-years away would be closer to Earth than the 4.3 million solar mass black hole at the Milky Way's center, located 26,000 light-years away. Besides the galactic center, this would be the only known case of numerous stars closely bound to a massive black hole.

The team aims to further characterize the black hole. While its mass is estimated to be at least 8200 solar masses, its exact mass and position are yet to be determined. They also plan to study the orbits of the fast-moving stars, requiring additional measurements of their line-of-sight velocities. The team has secured time with the NASA/ESA/CSA James Webb Space Telescope for this purpose and has other observatory proposals pending.

Omega Centauri was also highlighted in a recent data release from ESA's Gaia mission, featuring over 500,000 stars. "Even after 30 years, the Hubble Space Telescope with its imaging instruments is still one of the best tools for high-precision astrometry in crowded stellar fields, regions where Hubble can provide added sensitivity from ESA's Gaia mission observations," shared team member Mattia Libralato of the National Institute for Astrophysics in Italy (INAF), and previously of AURA for the European Space Agency during the time of this study. "Our results showcase Hubble's high resolution and sensitivity that are giving us exciting new scientific insights and will give a new boost to the topic of IMBHs in globular clusters."

Research Report:Fast-moving stars around an intermediate-mass black hole in Omega Centauri

Related Links
Hubble Space Telescope
Understanding Time and Space