The best evidence yet for an intermediate-mass black hole has been claimed by an international team of astronomers. Maximillian Häberle at the Max Planck Institute for Astronomy in Heidelberg and colleagues saw the gravitational effects of the black hole in long-term observations of the stellar cluster Omega Centauri. They predict that similarly-sized black holes could exist at the centres of other large, dense stellar clusters – which could explain why so few of them have been discovered so far.
Black holes are small and extraordinarily dense regions of space with huge gravitational fields that not even light can escape. Astronomers know of many stellar-mass black holes that weigh-in under about 100 solar masses. They are also are aware of supermassive black holes, which have 100,000s to billions of solar masses and reside at the centres of galaxies.
However, researchers know very little about the existence (or otherwise) of intermediate-mass black holes (IMBHs) in the 100–100,000 solar mass range. While candidate IMBHs have been spotted, no definite discoveries have been made. This raises questions about how supermassive black holes were able to form early in the history of the universe.`
Seeding supermassive growth
“One potential pathway for the formation of these early supermassive black holes is by the merger of intermediate mass ‘seed’ black holes,” explains Häberle. “However, the exact mass and frequency of these seeds is still unknown. If we study IMBHs in the present day, local, universe we will be able to differentiate between different seeding mechanisms.”
Häberle’s team examined the motions of stars within the globular cluster Omega Centauri, located around 17,000 light–years from Earth. Containing roughly 10 million stars, the cluster is widely believed to be the core of an ancient dwarf galaxy that was swallowed by the Milky Way. This would make it a prime target in the ongoing hunt for an IMBH within our own galaxy.
Häberle’s team analysed a series of images of Omega Centauri taken by the Hubble Space Telescope across a 20 years. By comparing the relative positions of the cluster’s stars in successive images, they identified stars that were moving faster than expected. Accelerated motion would be strong evidence that an IMBH is lurking somewhere in the cluster.
“This approach is not new, but we combined improved data reduction techniques with a much larger dataset, containing more than 500 individual images taken with the Hubble Space Telescope,” Häberle explains. “Therefore, our new catalogue is several times larger and more precise than all previous efforts.”
While some previous studies have presented evidence of an IMBH at the centre of Omega Centauri, the gravitational influence of unseen stellar-mass black holes could not be ruled out.
Seven speedy stars
Häberle’s team identified a total of seven stars at the very centre of Omega Centauri that appear to be moving much faster than the cluster’s escape velocity. Without some immense gravitational intervention, the researchers calculated that each of these stars would have left the centre of the cluster in less than 1000 years – a small blip on astronomical timescales – before escaping the cluster entirely.
Medium-sized black hole seen lurking in the Milky Way
“The best explanation why these stars are still around in the centre of the cluster is that a massive object is gravitationally pulling on them and preventing their escape,” Häberle claims. “The only object that can be massive enough is an intermediate-mass black hole with at least 8200 solar masses.”
The study makes Omega Centauri the best candidate in the Milky Way for having a IMBH. If confirmed, the IMBH will be the most massive black hole in the Milky Way after Sagittarius A* – the SMBH residing at our galaxy’s centre.
“To draw further conclusions and gain a statistical sample, we will now need to extend this research to other massive star clusters, where there might be still some hidden black holes,” Häberle says. The astronomers now hope that similar observations could soon be made using instruments including the Multi Unit Spectroscopic Explorer at the Very Large Telescope, and the James Webb Space Telescope’s Near-IR Spectrograph.
The research is described in Nature.
