One of the most sought-after objects in the Universe has just been detected right here in the Milky Way galaxy.
At the center of a tightly packed, spherical cluster of stars named Omega Centauri around 17,000 light-years away, astronomers have found evidence of an intermediate-mass black hole, tipping the cosmic scales at a mass equivalent to at least 8,200 Suns.
It's one of the best leads we have yet on these elusive beasts – black holes that fall in the mass range between star-mass black holes and the supermassive monsters that lurk at the hearts of galaxies. And it's the latest in a number of detections in globular clusters of stars – confirming that these strange assemblages are one of the best places to look.
"Here we report the observations of seven fast-moving stars in the central 3 arcsec (0.08 pc) of Omega Centauri," writes a team of astronomers led by Maximilian Häberle of the Max Planck Institute for Astronomy in Germany.
"The velocities of the fast-moving stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can be explained only by being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit of the black hole mass of about 8,200 solar masses, making this a good case for an intermediate-mass black hole in the local Universe."
Intermediate-mass black holes (IMBHs) are in short supply, at least as far as we have been able to gauge so far. These fall in a poorly defined mass range that usually sits between about 100 and 100,000 to a million solar masses.
On either side, we have stellar-mass black holes on the small end, those that have formed from the core collapse of a massive star and mergers of these black holes; and on the larger side are supermassive ones, millions to billions of times the mass of the Sun.
This presents a problem, because without IMBHs, there's no 'connective tissue' bridging the two mass ranges. Astronomers think that supermassive black holes can grow gradually from the slow accumulation and hierarchical mergers of stellar-mass black holes, but we'd need a lot more evidence for IMBHs to account for the number of supermassive black holes out there.
Globular clusters seem to be a good place to look. These are groups of stars that can number in the millions, all hanging out together in a roughly spherical structure, packed in like sparkly sardines. The Milky Way has around 150 known globular clusters, and their origin is a bit of a mystery.
But previous studies on globular clusters have found high concentrations of mass in their centers consistent with the mass ranges of intermediate-mass black holes. And, indeed, evidence that there might be such an object lurking therein.
Omega Centauri is thought to be the stripped core or nucleus of what was once a dwarf galaxy called the Gaia Sausage. It's roughly 150 light-years across, and contains around 10 million stars. Dwarf galaxies are like the smaller versions of full-sized galaxies, and it's possible that, rather than a supermassive black hole in their center, they revolve around IMBHs.
Now, a black hole is pretty hard to spot if it's just hanging out in space not doing anything, so searches for IMBHs in globular clusters and dwarf galaxies often turn to stellar kinematics – the study of the way stars move around a mass due to gravitational interactions. The most famous example is the stars orbiting the giant black hole at the center of the Milky Way galaxy, Sagittarius A* (Sgr A*).
Previously, studies examining the motion of stars in Omega Centauri have found evidence that there's an IMBH lurking therein. Those were more than a decade ago, and Häberle and his colleagues wanted to really get in there and see if they could narrow it down further.
Using 20 years of data collected using the Hubble Space Telescope – more than 500 images – they constructed an updated and much more detailed proper-motion catalog of the central region of Omega Centauri, looking for stars that appear to be moving as though influenced by a giant invisible mass.
In the very central region, they found some fast-moving stars very similar to those orbiting Sgr A*. And, by carefully studying their velocity and motion, they were able to determine the lower limit of the mass of the object they appear to be interacting with. At 8,200 solar masses, that's an IMBH, no matter which way you slice it.
In fact, the researchers say, an IMBH is the only plausible explanation for what they have found.
"This black hole provides an important data point in the study of black hole demographics in low-mass galaxies, along with other black holes that have been detected in more massive globular clusters and stripped [galactic] nuclei," they write in their paper.
"Moreover, this black hole provides the closest massive black hole and only the second after Sgr A* for which we can study the motion of multiple individual bound stellar companions."
This discovery, they say, suggests revisiting other globular clusters, and applying similar methodology to narrow down what secrets might be found therein.
The research has been published in Nature.
