One of the most sought-after objects in the universe has just been discovered in our Milky Way galaxy.
At the center of a densely packed globular cluster called Omega Centauri, about 17,000 light-years away, astronomers have found evidence for the existence of an intermediate-mass black hole. The mass of this black hole is at least equal to that of 8,200 suns in the universe.
It’s one of the best clues we have yet about these elusive beasts – black holes that fall in the mass range between stellar-mass black holes and the supermassive monsters lurking at the hearts of galaxies. And it’s the latest in a series of detections in globular clusters – confirming that these strange assemblages are among the best places to look.
“Here we report observations of seven fast-moving stars in the central 3 arcseconds (0.08 pct) 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 cluster, so their presence can only be explained by their being bound to a massive black hole. From the velocities alone, we can infer a firm lower limit on the mass of the black hole 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 rare, at least as far as we can tell. They fall into a poorly defined mass range that is usually between 100 and 100,000 to a million solar masses.
On both sides, we have, on the small side, stellar-mass black holes, which are formed by the collapse of the core of a massive star and by mergers of these black holes. On the larger side, we have supermassive black holes, which are millions to billions of times as massive as the sun.
This poses a problem, because without IMBHs there is no ‘connective tissue’ bridging the two mass ranges. Astronomers think that supermassive black holes can grow gradually through the slow accumulation and hierarchical mergers of stellar black holes, but we would need much more evidence for IMBHs to explain the number of supermassive black holes outside of them.
Globular clusters seem like a good place to look. These are groups of stars that can number in the millions, all packed together in a roughly spherical structure, like glistening sardines. The Milky Way has about 150 known globular clusters, and their origins are a bit of a mystery.
But previous studies of globular clusters have found high concentrations of mass in their centers that are consistent with the mass ranges of intermediate-mass black holes. And indeed, evidence that such an object might lurk within them.
Omega Centauri is thought to be the stripped core or nucleus of what was once a dwarf galaxy called the Gaia Sausage. It is about 150 light-years across and contains about 10 million stars. Dwarf galaxies are like smaller versions of full-fledged galaxies and it is possible that, instead of having a supermassive black hole at their center, they orbit IMBHs.
Now, a black hole is pretty hard to spot if it’s just hanging out in space doing nothing, so searching for IMBHs in globular clusters and dwarf galaxies often focuses on stellar kinematics – the study of how stars move around a mass through gravitational interactions. The best-known example is the stars orbiting the giant black hole at the center of the Milky Way, Sagittarius A* (Sgr A*).
Previous studies that looked at the motion of stars in Omega Centauri had found evidence that an IMBH lurked there. That was more than a decade ago, and Häberle and his colleagues wanted to really dig in and see if they could constrain it further.
Using 20 years of data collected with the Hubble Space Telescope – more than 500 images – they constructed an updated and much more detailed catalogue of the proper motion of Omega Centauri’s central region, looking for stars that appear to move as if influenced by a gigantic unseen mass.
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In the central region, they found a number of fast-moving stars that looked very much like those orbiting Sgr A*. And by studying their speed and motion in detail, they were able to determine the lower limit on the mass of the object they seemed to be interacting with. At 8,200 solar masses, that’s an IMBH by any means you look at it.
The researchers say an IMBH is the only plausible explanation for what they found.
“This black hole provides an important data point in the study of the demography of black holes in low-mass galaxies, along with other black holes detected in more massive globular clusters where mass has been removed. [galactic] “cores,” they write in their article.
“Moreover, this black hole is the closest massive black hole and only the second after Sgr A* that allows us to study the motion of multiple individual stellar companions.”
This discovery, they say, suggests we should re-examine other globular clusters and apply similar methodology to uncover what secrets lie hidden within them.
The research was published in Nature.