Milestone as intermediate-mass black hole found in Omega Centauri

Astronomers have used more than 500 images from the Hubble Space Telescope to detect seven fast-moving stars in the innermost region of Omega Centauri, the largest and brightest globular cluster in the sky. These stars provide compelling new evidence for the presence of an intermediate-mass black hole (IMBH) — a long-sought ‘missing link’ from the early stages of galaxy evolution — which has been published in the journal Nature.

Black holes are one of the most extreme environments humans are aware of, and so they are a testing ground for the laws of physics and our understanding of how the universe works. Most known black holes are either extremely massive, like the supermassive black holes that lie at the cores of large galaxies, or relatively lightweight, with a mass less than 100 times that of the Sun. But while our current picture of galaxy evolution posits that the earliest galaxies should have had intermediate-sized central black holes, which would have grown over time as those galaxies evolved, there has been no definite detection of such an intermediate-mass black hole until now.

This is where Omega Centauri — which is located 17,700 light-years away, just above the plane of the Milky Way — comes in. If it was once the core of a separate galaxy, which then merged with the Milky Way and lost all but its central batch of stars in the process, the remaining galactic core and its central black hole would be ‘frozen in time’: there would be no further mergers and no way for the central black hole to grow. The black hole would be preserved at the size it had when Omega Centauri was swallowed up by the Milky Way, providing a glimpse of the missing link between early low-mass black holes and the later supermassive black holes.

Nadine Neumayer, a group leader at the Max Planck Institute for Astronomy, partnered with Anil Seth from the University of Utah to settle the matter of the black hole’s existence once and for all. If they were able to identify fast-moving stars around the centre of Omega Centauri, that would be the proverbial smoking gun, as well as a way of measuring the black hole’s mass.

The search became the task of Maximilian Häberle, a PhD student at the Max Planck Institute. Häberle led the creation of an enormous catalogue for the motions of stars in Omega Centauri, measuring the velocities of 1.4 million of the cluster’s 10 million stars by studying over 500 Hubble images of the cluster. Most of these images had been produced over a period of two decades for the purpose of calibrating Hubble’s instruments, rather than for scientific use — but with their ever-repeating views of Omega Centauri, they turned out to be the ideal dataset for the team’s research efforts.

“We discovered seven stars that should not be there,” Häberle said. “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,” Häberle continued. The images do not indicate any visible object at the inferred location of that central mass, as one would expect for a black hole.

These three panels zero in on the location of the IMBH candidate in the Omega Centauri globular cluster, with the likely location of the black hole circled in the third panel. The line scale in the third panel is equivalent to 0.1 light-years.

The research team now plans to characterise the black hole, as its exact mass and its precise position are not fully known, as well as to study the orbits of the fast-moving stars through additional measurements of the respective line-of-sight velocities. The team has been granted time with the James Webb Space Telescope to do just that, and also has other pending proposals to use other observatories.

“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,” said team member Mattia Libralato, currently based at Italy’s National Institute for Astrophysics (INAF).

“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.”

Images credit: ESA/Hubble & NASA, M Häberle (MPIA).