One of the most sought after objects in the Universe has just been discovered right here in the Milky Way galaxy.
At the center of a strongly spherical star cluster called Omega Centauri, some 17,000 light-years away, astronomers have found evidence of an intermediate-mass black hole, tipping the cosmic scale at a mass equal to at least 8,200 Suns.
It’s one of the best leads we have yet for these elusive beasts—black holes that fall in the mass interval between stellar-mass black holes and the supermassive monsters that lurk in the hearts of galaxies. And it’s the latest in a number of discoveries in globular star clusters – confirming that these strange clusters are one of the best places to look.
“Here we report observations of seven fast-moving stars within 3 central arcs (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 escape velocity of the central star cluster, so their presence can only be explained by being associated with a massive black hole. From the velocities alone, we can we infer a lower black hole mass limit 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’ve been able to estimate so far. These fall in a poorly defined mass range that typically falls between about 100 and 100,000 to a million solar masses.
On either side, we have stellar-mass black holes at the small end, those formed by the collapse of the core of a massive star and the merging of these black holes; and on the larger side are the supermassive ones, millions to billions of times the mass of the Sun.
This presents a problem, because without IMBHs, there is no ‘connective tissue’ spanning the two mass strings. Astronomers think that supermassive black holes can grow gradually by slow accretion and hierarchical mergers of stellar-mass black holes, but we would need much more evidence for IMBHs to calculate the number of supermassive black holes out there.
Globular clusters seem to be a good place to look. These are clusters of stars that can number in the millions, all hanging together in a roughly spherical structure, packed like shiny 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 at their centers consistent with the mass range of intermediate-mass black holes. And, indeed, evidence that there may be such an object lurking there.
Omega Centauri is thought to be the core, or bare core, of what was once a dwarf galaxy called Sausage Gaia. It is approximately 150 light-years across and contains about 10 million stars. Dwarf galaxies are like smaller versions of full-sized galaxies, and it is possible that, instead of a supermassive black hole at their center, they revolve around IMBHs.
Now, a black hole is hard enough to spot if it’s just hanging out in space and doing nothing, so searches for IMBHs in globular clusters and dwarf galaxies often turn to stellar kinematics—the study of how how stars move around a mass due to gravitational interactions. The most famous example is the stars orbiting the supermassive 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 is an IMBH lurking there. 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 built an updated and much more detailed catalog of the proper motion of Omega Centauri’s central region, looking for stars that appear to be moving. as if influenced by a giant invisible mass.
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In the very central region, they found several fast-moving stars very similar to those orbiting Sgr A*. And by carefully studying their speed 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 found.
“This black hole provides an important data point in the study of black hole demography in low-mass galaxies, along with other black holes that have been discovered in more massive globular clusters and stripped. [galactic] core”, they write in their paper.
“Furthermore, 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 associated stellar companions.”
This discovery, they say, suggests revisiting other globular clusters and applying similar methodology to narrow down the secrets that may be found there.
The research was published in Nature.