SALT LAKE CITY — In a galaxy that spans many light-years and is more than 13 billion years old, it stands to reason that major astronomical discoveries are rare, to say the least.
Omega Centauri, a global cluster of millions of stars, is nestled in the Milky Way. The stars are so close to the center that it becomes impossible to distinguish individual stars. From southern latitudes, they are visible only as a tiny dot in the night sky.
But within that cluster lies something astronomers have been searching for and debating for nearly a decade. A new study led by researchers from the University of Utah and the MacPlanck Institute for Astronomy has revealed it: Omega Centauri harbors a central black hole.
“This is a once-in-a-career finding. I’ve been excited about it for nine months. Every time I think about it, I can’t sleep,” said Anil Seth, an associate professor of astronomy at the U. and co-principal investigator on the study.
‘On the level of Bigfoot’
The research, published Wednesday in the journal Nature, explained that black holes have different masses.
Common black holes are stellar-mass black holes, which range in mass from one to several tens of solar masses, and supermassive black holes, with masses up to billions of suns.
Even more striking and elusive than ever before are the intermediate-mass black holes, the type discovered by the research team.
“These medium-sized black holes are kind of on the level of Bigfoot. Spotting them is like finding the first evidence of Bigfoot — people freak out,” said Matthew Whittaker, a graduate student at the U. and co-author of the study.
‘Needle in a haystack’
Omega Centauri appears to be the core of a small, isolated galaxy whose evolution was aborted when it was swallowed up by the Milky Way, the paper said. The current state of galaxy evolution suggests that these earliest galaxies should have had medium-sized central black holes that would have grown over time.
But how do you find one?
Seth and Nadine Neumayer, group leader at the Max Planck Institute and principal investigator of the study, began research in 2019 into how the formation history of Omega Centauri could be better understood.
They found that if they found fast-moving stars around the center of the cluster, they could finally answer questions about the cluster’s central black hole by measuring the black hole’s mass.
This search for the stars fell into the lap of Maximilian Häberle, a doctoral student at the Max Planck Institute. Häberle led the development of a giant catalogue of the motions of stars in Omega Centauri, measuring the velocities of 1.4 million stars by studying more than 500 Hubble images of the cluster.
The problem here was that most of the images Häberle had at his disposal were intended to calibrate Hubble’s instruments, not to make groundbreaking scientific discoveries.
Yet this unintended dataset of over 500 images served its purpose.
“Looking for fast stars and documenting their motion was like looking for a needle in a haystack,” Häberle said. In the end, not only did Häberle have the most complete catalogue yet of the motion of stars in Omega Centauri, he also found seven needles in his archival haystack — seven fast-moving stars in a small region in the center of Omega Centauri.
The discovery
The work did not end with finding these seven stars, however. With seven stars, all with different velocities and directions of motion, the researchers were able to separate the different effects and establish that there is in fact a central mass in Omega Centauri, with the mass of at least 8,200 suns.
Furthermore, the images do not suggest that there are any visible objects at the location of that central mass, as you would expect for a black hole.
And more analysis led to more good news for the team. As the paper explained, a single fast star in the image might not belong to Omega Centauri. It could be a star outside the cluster that happens to pass directly behind or in front of Omega Centauri’s center. However, the observations of seven such stars cannot be a coincidence and leave no room for any explanation other than the presence of a black hole.
Checkmate.
Moving forward
The team now plans to build on its monumental findings by further investigating the center of Omega Centauri. Seth is leading a project that has received approval to use the James Webb Space Telescope to measure the motion of the superfast stars toward or away from Earth.
As future instruments roll out that can pinpoint the positions of stars even more precisely than Hubble, the goal will be to determine how the stars accelerate and how their orbits curve. That project, however, will fall into the hands of future generations of researchers.
However, this discovery confirms that Omega Centauri is the core region of what was once a galaxy that was engulfed by the Milky Way billions of years ago.
For those interested in hearing directly from the researchers, Seth will present the team’s findings on August 8 at 7 p.m. ET at the Clarke Planetarium IMAX theater in Salt Lake City. In the meantime, the full study is available online.
“I think extraordinary claims require extraordinary evidence. This is really, really extraordinary evidence,” Seth said.