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 globular star cluster called Omega Centauri some 17,000 light-years away, astronomers have found evidence of an intermediate-mass black hole that tips the cosmic scales at a mass equivalent to at least 8,200 Suns.
It’s one of the best clues we have to these elusive creatures—black holes that fall in the mass range between stellar-mass black holes and the supermassive monsters that lurk in the hearts of galaxies. And it’s the latest in a series of detections in globular clusters – confirming that these strange groupings are one of the best places to look.
“Here we report 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 Häberl 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 hard lower limit of black hole mass of about 8,200 solar masses, which is 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 measure so far. These fall into an ill-defined mass range that usually lies 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 created by the collapse of the core of a massive star and the merger of these black holes; and on the larger side, they are supermassive, millions to billions of times the mass of the Sun.
This presents a problem because without the IMBH there is no “connective tissue” to bridge these two mass ranges. Astronomers believe that supermassive black holes can grow gradually from the slow accretion and hierarchical merger of stellar-mass black holes, but we would need much more evidence for IMBHs to explain the number of supermassive black holes out there.
Globular clusters seem like a good place to look. These are groups of stars that may number in the millions, all hanging together in a roughly spherical structure, packed like glittering sardines. The Milky Way has around 150 known globular clusters, and their origins are a bit of a mystery.
However, previous studies of globular clusters have found high concentrations of matter in their centers that correspond to the mass ranges of intermediate-mass black holes. And indeed proof that such an object could be hidden in it.
Omega Centauri is thought to be the stripped core, or core, of what was once a dwarf galaxy called the Gaia Sausage. It is roughly 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 rather than having a supermassive black hole at their center, they revolve around an IMBH.
Now, a black hole is pretty hard to detect if it’s just hanging around in space doing nothing, so the search for IMBHs in globular clusters and dwarf galaxies often turns to stellar kinematics—the study of how stars move around matter due to gravity. interaction. The most famous example is the stars orbiting the supermassive black hole at the center of the Milky Way galaxy, Sagittarius A* (Sgr A*).
Earlier studies examining the motion of the stars in Omega Centauri found evidence of an IMBH hiding within them. That was more than a decade ago, and Häberle and his colleagues wanted to really get in there and see if they could narrow it down any further.
Using 20 years of data collected by the Hubble Space Telescope – more than 500 images – they constructed an updated and much more detailed catalog of the proper motion of the central region of Omega Centauri, looking for stars that appear to be moving as if affected by a giant invisible mass.
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In the very central region, they found some 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 with which they appear to be interacting. At 8,200 solar masses, it’s an IMBH no matter which way you slice it.
In fact, the IMBH is the only plausible explanation for what they found, the researchers say.
“This black hole provides an important data point in studying the demography of black holes in low-mass galaxies, along with other black holes that have been detected in more massive globular clusters and removed. [galactic] cores,” they write in their paper.
“In addition, this black hole provides the closest massive black hole, and only the second one after Sgr A*, for which we can study the motion of multiple individual bound stellar companions.”
The discovery, they say, suggests revisiting other globular clusters and using a similar methodology to narrow down the secrets they might hold.
The research was published in Nature.