There may be a strange, in-between kind of black hole lurking at the heart of a star cluster not too far away.
At the center of the Omega Centauri cluster — a dense ball of stars about 17,000 light years away from Earth — a handful of stars are moving so quickly that they should be breaking free of the cluster’s gravity. But instead, they seem to be caught in the gravitational grasp of an unseen mass. Max Planck Institute for Astronomy astrophysicist Maximilian Häberle and his colleagues say that hidden mass is probably a black hole, too small to be the kind of supermassive leviathan that feasts at the heart of our galaxy, but too large to be the mere remains of a collapsed star. If they’re right, the black hole in Omega Centauri could be a missing link in the evolutionary history of black holes.
Häberle and his colleagues published their work in the journal Nature.
A Cosmic Missing Link
Over the past 20 years, the venerable Hubble Space Telescope has imaged Omega Centauri, which has more than 10 million stars clumped into a ball just 150 light years wide, about 500 times. Häberle and his colleagues used those images to trace how about 150,000 stars in the cluster moved over time (because all the stars in a cluster move around a shared center of gravity, and they occasionally pull or nudge at each other, too). The astronomers spotted 7 stars near the center of the cluster moving fast enough to escape the cluster’s gravity. But somehow, they weren’t actually escaping.
The fastest of the stars seemed to be zipping around the core of the cluster at around 250,000 miles an hour. That should have been fast enough to overcome even the immense pull of Omega Centauri’s gravity, based on what astronomers think they know about the mass of all the stars and gas in the cluster.
“Their velocities would cause them to leave the central region in about 1,000 years, and then eventually escape the cluster,” write Häberle and his colleagues in their recent paper. After checking and double-checking their data, the astronomers were left with only one explanation that made sense: Something massive must be lurking at the heart of Omega Centauri, pulling those seven stars into faster-moving orbits and not letting them escape.
That something, Häberle and his colleagues say, is probably something astronomers have long imagined but never actually seen: an intermediate-mass black hole. And it could be a crucial clue to the still-murky origins of supermassive black holes.
You Say “Mid” Like It’s a Bad Thing
When an extremely massive star — something the size of Betelgeuse, for example — burns up the last of its fuel and collapses under its own ponderous mass, it leaves behind a bizarre object called a black hole. These so-called “stellar mass” black holes are between 5 and 150 times the mass of our Sun, packed into a point in spacetime so tiny that it doesn’t actually have a measurable diameter: a singularity. The gravitational pull a black hole exerts is powerful enough to trap anything that ventures too close, even light.
But stellar mass black holes are tiny and cute compared to the behemoths that lurk at the centers of galaxies: supermassive black holes weigh in at more than 100,000 times the mass of our Sun (and the one at the center of the Milky Way is about 4 million times more massive than the Sun), and they feast on whole stars, sometimes handfuls at a time. Physicists are still trying to figure out how supermassive black holes originally formed.
One theory suggests that after the first stars in the universe died, their small stellar mass black holes collided and merged into bigger black holes until they eventually accumulated enough mass to be supermassive black holes. Another theory suggests that the first supermassive black holes formed from larger “seeds,” like entire nebulae that collapsed under their own mass into something several thousand times the mass of a star, but nowhere near supermassive. Then, over time, those black holes collided and merged until, eventually, voila, supermassive black holes.
Either way, physicists are missing a key piece of the story: black holes bigger than 150 times our Sun’s mass, but not big enough to count as supermassive black holes. These cosmic missing links are called intermediate mass black holes, and they could help fill in the blanks of how supermassive black holes formed — either by making a connection between stellar mass black holes and their supermassive cousins, or by revealing what the big “seeds” of modern supermassive black holes looked like.
The trouble with the missing link, though, is that it’s… well, missing. Astronomers have only spotted a handful of things that even might be intermediate-mass black holes.
What’s Next?
Häberle and his colleagues calculate that the unseen mass in the center of Omega Centauri should be at least 8,200 times the mass of our Sun, and some of their models suggest it could even be as massive as 21,100 Suns. The seven fast-moving stars zipping around it are skimming daringly close, just a quarter of a light year from the center of the cluster – and, apparently, from the edge of the black hole’s event horizon.
“These fast-moving stars are a predicted consequence of an intermediate mass black hole,” write Häberle and his colleagues.
To confirm that that’s the case — and narrow down exactly how massive this alleged intermediate-mass black hole is and where it lives — astronomers will need a lot more information.
“A planned study using the James Webb Space Telescope will attempt exactly this,” write McGill University astrophysicist Darryl Haggard and San Francisco State University astrophysicist Adrienne Cool, who weren’t involved in the recent study, in a paper commenting on it. That study will also track the stars’ movements in three dimensions; the Hubble data could only track how they moved in two dimensions across the telescope’s field of view. That will give astronomers a more precise look at how fast they’re moving, which will help narrow down the black hole’s mass.
Häberle and his colleagues also want to search for evidence of other intermediate mass black holes in places like Omega Centauri: dense globular clusters of stars. Omega Centauri was a prime place to search for an intermediate mass black hole partly because it’s relatively close to Earth (we did say relatively; space is big), but also because astronomers are fairly sure that it’s all that remains of a dwarf galaxy that our Milky Way swallowed up billions of years ago. Omega Centauri would have been the bright, dense core of that tiny galaxy (amusingly known as Gaia-Enceladus-Sausage), so it should contain exactly the kind of intermediate black hole that — in theory — merged to form supermassive black holes.
