Omega Centurai’s Monster Black Hole

Omega Centauri is a familiar sight to southern watchers. On a clear night, and from dark enough skies, you can just make it out with the naked eye. It is a sight to behold to amateur astronomers, astrophotographers, and curious enthusiasts who stumble across it with their telescopes and imaging equipment. A collection of yellow and white stars (with the occasional blue and red) all silently hovering around a central focus point – like bees, swirling around a hive, yet frozen in time. 

Scientists have also been observing Omega Centauri (or 'Omega Cen' as they call it) across the electromagnetic spectrum for years. While it's not the only globular cluster in the Milky Way, it stands out due to its unique mass, chemical composition, stellar population, and core density. Many believe that Omega Cen is the remnant core of a former dwarf galaxy that the Milky Way has gradually cannibalised over the ages.

With a reported cluster mass of about 4 million times that of the Sun, astronomers have long speculated whether, like other galaxies, Omega Cen harbours a massive black hole at its core. Such a black hole would fall between the familiar stellar-mass black holes (formed when massive stars die) and the supermassive black holes at the centres of galaxies, which contain millions to billions of solar masses. This middle ground is occupied by Intermediate-Mass Black Holes (IMBH) - a population that has so far evaded astronomers.

However, an international team of scientists - led by PhD Student Maximilian Häberle from the Max-Planck Institute for Astronomy, has uncovered the strongest evidence yet for an Intermediate-Mass Black Hole (IMBH) at the heart of Omega Cen. This groundbreaking study involved analysing and cataloguing 1.4 million stars within the cluster and comparing them against theoretical models.

One of the co-authors, Associate Professor Holger Baumgardt from the University of Queensland, played a pivotal role in this research, which has now announced its remarkable findings commenting on how these significant results now provide strong evidence for the existence of IMBH and a formation channel for supermassive black holes.

“At the moment there are two competing ideas about their [supermassive black hole] formation, they could have either grown over time out of stellar mass black holes through accreting gas (and stars and maybe even dark matter) or they formed directly either in the Big Bang or by the collapse of (so far completely hypothetical) supermassive stars,” he said. “Our discovery of a black hole in the intermediate mass regime could indicate that it is the slow growth of stellar mass black holes which forms supermassive ones.”

“Led by the Max-Planck-Institut für Astronomie in Germany, our team searched for fast-moving stars expected to exist near concentrated masses, like black holes,” Dr Baumgardt said.

“I did the theoretical modelling and investigated if the very fast stars can be explained without an IMBH. But the answer is a definite No. In the absence of an IMBH, the maximum velocity a star can have without being unbound to Omega Cen is about 65 km/sec.”

These new findings, published in the journal Nature, detail the analysis of data collected by the Hubble Space Telescope over the past twenty years, focusing on the stellar population of Omega Centauri. This was not a dedicated study; instead, the data (over 500 images) were gathered as the space-based telescope used Omega Centauri for calibration. This serendipitous accumulation of images provided a treasure trove of data for analysis.

“These images were originally produced to calibrate the Hubble’s instruments rather than for scientific use, but they turned out to be the ideal data set for our research and proved to be invaluable,” Dr Baumgardt said.

“The process was to download the individual images the HST has taken over time from the HST data archive, measuring the exact position of the stars on these images, and then deriving the motion of each star over time. The process is hard though, there are only very few people in the world which have the expertise to do this with the required precision.”

The IMBH itself cannot be directly observed (this has only ever happened using the Event Horizon Telescope for two supermassive black holes – one at the heart of the Milky Way (Sgr A*) and the other at the centre of the M87 Galaxy). Instead, the team of astronomers tracked the motion of the stars around the central region of Omega Centauri, where the massive object is expected to reside. They discovered that the stars were being flung around at such enormous velocities that they should break free of the cluster’s gravity and sail off into space. However, an invisible object with a mass of 20,000 Suns keeps them in orbit.

“We now know that there is an IMBH in the [Omega Cen] core. Our data also shows that the cluster itself is very complex, there are multiple generations of stars and the kinematics of the stars of each generation is different, for example in terms of rotation,” he said. “It really looks as if Omega Cen was the core of a dwarf galaxy into which gas spiralled in at different times and from different directions and formed successive generations of stars. Probably during each of these events, some of the gas was also swallowed by the forming intermediate-mass black hole and this is how it acquired its present-day mass.”

These findings pave the way for further studies and analysis of the IMBH in Omega Centauri. Future spectroscopic studies of these fast-moving stars using ground-based instruments like the Very Large Telescope array in Chile or the James Webb Space Telescope (JWST) could provide even more detailed insights. By measuring the acceleration of these stars over long-term baselines and analysing their spectra, astronomers hope to uncover fascinating details about the dynamics surrounding the IMBH.

“I think this will invigorate the field and lead to a lot of new research in this area,” he said.

“We can begin to speculate how this black hole formed, how it’s related to other massive black holes astronomers have found, and where other intermediate-mass black holes might exist,” said Dr Baumgardt.

Another promising method involves using millisecond pulsars (MSPs) in orbit around the central massive compact object. MSPs are incredibly stable and function as cosmic clocks. When they are in a strong gravitational field, slight variations in their ticks can help reveal the parameters of the IMBH. 

While no MSPs have been found in this specific region to date, recent studies have discovered 18 new MSPs around the core of Omega Centauri. With the increased sensitivity of upcoming radio telescopes like the Square Kilometre Array (SKA), the holy grail of radio astronomy might finally be within reach – a millisecond pulsar in orbit around a massive black hole.

“It’s a thrilling time to be an astronomer and we’re excited to see where our discovery will lead.”

Video credit: T. Müller, M. Häberle (MPIA)