Black hole debate settled? Stellar-mass black holes found at the heart of the Milky Way's largest star cluster

Could a decades-long debate about the mysterious movements of stars in Omega Centauri, the largest star cluster in the Milky Way, finally be resolved?  

Omega Centauri is a massive star cluster with nearly ten million stars located in the constellation Centaurus. For a long time, researchers have noticed that the velocities of stars moving near the centre of Omega Centauri were higher than expected. But it wasn't clear whether this was caused by an "intermediate mass" black hole (IMBH), weighing a hundred thousand times the mass of the Sun, or a cluster of "stellar mass" black holes, each weighing just a few times the mass of the Sun. 

A cluster of black holes is expected to form at the centre of Omega Centauri as a result of stellar evolution. But astronomers thought that most of them would be ejected by slingshot interactions with other stars. As such, an IMBH hole started to look more and more like the favoured solution. This seemed even more likely when new evidence recently emerged of fast-moving stars near the centre of Omega Centauri that may require interactions with an IMBH to reach such high velocities. 

Intermediate mass black holes (IMBHs) are exciting to astronomers because they may be the "missing link" between stellar mass black holes and supermassive black holes. Stellar-mass black holes form from the death of massive stars and have already been found via a variety of different techniques. Supermassive black holes are found at the centres of large galaxies and can weigh millions to billions of times the mass of the Sun. We do not currently know how supermassive black holes form or whether they begin their lives as stellar mass black holes. Finding an IMBH could solve this cosmic puzzle. 

The new research involving the University of Surrey looked afresh at the anomalous velocities of stars at the centre of Omega Centauri, but this time, it used a new piece of data. The researchers combined the anomalous velocity data with new data for the accelerations of pulsars for the first time. Pulsars, like black holes, are formed from dying stars. Weighing up to twice the mass of the Sun, they are just 20km across and can spin up to 700 times a second. As they spin, they emit radio waves along their spin axis, processing like a spinning top. The radio beam sweeps past the Earth like a lighthouse, allowing us to detect them. 

Pulsars are natural clocks, almost as accurate as atomic clocks on Earth. By carefully measuring the change in the rate of their spin, astronomers can calculate how the pulsars are accelerating, directly probing the gravitational field strength at the centre of Omega Centauri. Combining these new acceleration measurements with the stellar velocities, researchers from Surrey, the Instituto de Astrofísica de Canarias (IAC, Spain) and the Annecy-le-Vieux Laboratoire de Physique Théorique LAPTh in Annecy (France) were able to tell the difference between an IMBH and a cluster of black holes, favouring the latter. 

Professor Justin Read, co-author of the study from the University of Surrey, said: 

"The hunt for elusive intermediate-mass black holes continues. There could still be one at the centre of Omega Centauri, but our work suggests that it must be less than about six thousand times the mass of the Sun and live alongside a cluster of stellar mass black holes. There is, however, every chance of us finding one soon. More and more pulsar accelerations are coming, allowing us to peer into the centres of dense star clusters and hunt for black holes more precisely than ever before." 

Andrés Bañares Hernández, lead author of the study from IAC, said: 

"We have long known about supermassive black holes at galaxy centres and smaller stellar-mass black holes within our own galaxy. However, the idea of intermediate-mass black holes, which could bridge the gap between these extremes, remains unproven." 

"By studying Omega Centauri – a remnant of a dwarf galaxy – we have been able to refine our methods and take a step forward in understanding whether such black holes exist and what role they might play in the evolution of star clusters and galaxies. This work helps resolve a two-decade-long debate and opens new doors for future exploration." 

"The formation of pulsars is also an active field of study because a large number of them have recently been detected. Omega Centauri is an ideal environment to study models of their formation, which we have been able to do for the first time in our analysis."