SALT LAKE CITY — In a galaxy spanning many light years and more than 13 billion years old, it makes sense that major astronomical discoveries happen infrequently — and that’s putting it mildly.
Nestled in the Milky Way is Omega Centauri, a global cluster of millions of stars—so dense toward the center that individual stars become impossible to distinguish—visible only as a tiny speck in the night sky from southern latitudes.
But within that cluster is something astronomers have been searching for and debating for nearly a decade, and something a new study led by researchers at the University of Utah and the Mac Planck Institute for Astronomy has found: Omega Centauri is home to a central black hole .
“This is a once-in-a-career discovery. I’ve been excited about it for nine straight months now. Every time I think about it, I have trouble sleeping,” said Anil Seth, associate professor of astronomy at the U. and co-principal investigator of the study.
“At the level of the Big Step”
The study, which was published in the journal Nature on Wednesday, explains that black holes come in different mass ranges.
Common black holes include stellar black holes, ranging from one to a few tens of solar masses, and supermassive black holes, with masses up to billions of suns.
More famously elusive without definitive detections – until now – are the intermediate-mass black holes discovered by the research team.
“These intermediate-mass black holes are somewhat at the level of the Big Step. Spotting them is like finding the first evidence of the Big Step — people are going to go crazy,” 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 nucleus of a small separate galaxy whose evolution was interrupted when it was swallowed by the Milky Way, the paper states. The current state of galaxy evolution suggests that these earliest galaxies should have had intermediate-sized central black holes that would have grown over time.
But how do you find one?
Seth and Nadine Neumeier, group leader at the Max Planck Institute and principal investigator of the study, first began investigating how to better understand the formation history of Omega Centauri in 2019.
They realized that if they found fast-moving stars around its center, they could finally solve the question around the cluster’s central black hole by measuring the black hole’s mass.
This search for the stars landed in the lap of Maximilian Haberle, a PhD student at the Max Planck Institute. Heberle led the work on developing a massive catalog of the motions of stars in Omega Centauri, measuring velocities for 1.4 million stars by studying more than 500 Hubble images of the cluster.
The challenge with this was that most of the images Haeberle had were taken to calibrate Hubble’s instruments, not to help ground-breaking scientific discoveries.
Still, with over 500 images, this unintended dataset served its purpose.
“Searching for high-velocity stars and documenting their motion has been the proverbial search for a needle in a haystack,” Haberle said. In the end, Haberle not only had the most complete catalog to date of the motion of stars in Omega Centauri, but also found seven needles in his archival haystack—seven fast-moving stars in a small region at the center of Omega Centauri.
The discovery
However, the work did not end with finding these seven stars. With seven stars, all with different speeds and directions of motion, the researchers were able to separate the various effects and determine that there is in fact a central mass in Omega Centauri, with a mass of at least 8,200 suns.
Furthermore, the images do not suggest any visible objects at the location of this central mass, as expected for a black hole.
And more analysis led to more good news for the team. As the paper explains, a single high-velocity star in the image may not belong to Omega Centauri. It could be a star outside the cluster that happens to pass just behind or in front of the center of Omega Centauri. The observations of seven such stars, on the other hand, cannot be accidental and leave no room for other explanations than the presence of a black hole.
Chess and Checkmate.
I’m moving forward
The team now plans to build on their monumental findings by further exploring the center of Omega Centauri. Seth led a project that won approval to use the James Webb Space Telescope to measure the motion of high-velocity stars to or from Earth.
With the advent of future instruments that could determine the positions of stars with even greater precision than Hubble, the goal is to determine how stars accelerate and how their orbits curve – but that project will fall into the hands of future generations of researchers .
Still, this discovery builds the case for Omega Centauri as the core region of what was once a galaxy swallowed up by the Milky Way billions of years ago.
For people interested in hearing directly from the researchers, Seth will present the team’s findings on August 8 at 7:00 pm at the Clarke Planetarium IMAX theater in Salt Lake City. In the meantime, the full study can be found online.
“I think extraordinary claims require extraordinary evidence. This is really, really extraordinary evidence,” Seth said.
