Using the Hubble Space Telescope, astronomers have discovered the largest black hole Earth has ever seen, a cosmic Titan “frozen in time.”
An example of an elusive “intermediate-mass black hole,” this object could serve as a missing link in understanding the connection between stellar-mass and supermassive black holes. The black hole appears to have about 8,200 suns, which is significantly larger than stellar-mass black holes, which are 5 to 100 times the mass of the Sun, and much less massive than the aptly named supermassive black holes. The mass of the Sun ranges from millions to billions. The closest stellar-mass black hole scientists have found is called Gaia-BH1, and it’s 1,560 light-years away.
The newly discovered intermediate-mass black hole, on the other hand, resides in a magnificent cluster of about ten million stars called Omega Centauri, located 18,000 light-years from Earth.
Interestingly, the fact that the “frozen” black hole appears to have stunted its growth supports the idea that Omega Centauri is the remains of an ancient galaxy that was cannibalized by our own galaxy.
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This would suggest that Omega Centauri is actually the core of a small, isolated galaxy whose evolution was cut short when the Milky Way swallowed it. If this event never happened, this interstellar black hole would be located in the Milky Way’s own supermassive black hole, Sagittarius A* (Sgr A*), 4.3 million times the mass of the Sun. It is 27,000 light years from Earth.
Searching for the missing
Scientists have known for some time that not all black holes are created equal. Although stellar-mass black holes are known to form through the collapse of stars with masses at least eight times that of the Sun, supermassive black holes must have a different origin. Because no star is big enough to collapse and survive Millions Many times bigger than the Sun.
Therefore, scientists propose that supermassive black holes are born and grow due to the merging chains of progressively larger and larger black holes. This has been demonstrated by detecting ripples in spacetime called gravitational waves emanating from black hole mergers.
This process of black hole mergers and growth, combined with the mass gap between stellar-mass black holes and supermassive black holes, means that there should be a population of intermediate-mass black holes.
Nevertheless, these intermediate-mass black holes, with masses ranging from a few hundred to a few thousand times that of the Sun, seem to have eluded detection for the most part. That’s because, like all black holes, these medium-sized cosmic titans are marked by outer boundaries called event boundaries.
The event horizon is the point at which the black hole’s gravitational influence becomes so immense that even light is not fast enough to escape it. Therefore, black holes can only be seen in light if they are surrounded by material to feed on, either glowing as they heat up, or tearing apart and feeding on an unlucky star in a so-called “tidal disruption event” (TDE).
Interstellar black holes, like those in Omega Centauri, are not surrounded by much matter and food.
That means astronomers have to be a little more crafty when hunting for such black holes. They exploit the gravitational effects these voids have on matter such as stars orbiting them or light passing through them. This new discovery team used the previous method.
A fast star
The hunt for this interstellar black hole began in 2019, when Nadine Neumeier of the Max Planck Institute for Astronomy (MPIA) and Anil Seth of the University of Utah designed a research project to improve our understanding of Omega Centauri’s formation history.
Specifically, researchers and collaborator Maximilian Heberle, an MPIA Ph.D. The student wanted to find fast-moving stars in Omega Centauri, demonstrating that the star cluster has a massive, dense, or compact “central engine” black hole. A similar method was used to determine the mass and size of Sgr A* using the number of fast-moving stars in the center of the Milky Way.
Haeberle and his team used more than 500 Hubble images of this star cluster to create an extensive database of the motions of the stars in Omega Centauri, measuring the velocities of about 1.4 million stars. This ever-repeating sighting of Omega Centauri was conducted by Hubble not out of scientific interest, but to calibrate its instruments, providing better data for the team’s work.
“Looking for supernovae and documenting their motion is like looking for a needle in a haystack,” Heberle said. The team ultimately didn’t find one, though Seven “Needle-in-the-haystack stars,” all moving at rapid speeds in a small area at the center of Omega Centauri.
The fast speed of these stars is due to the concentrated mass nearby. If the team found only one fast star, it would be impossible to determine whether its speed was the result of a large and close central mass, or whether the star was simply a runaway speeding along a straight path – or nothing. central mass.
Finding and measuring the different velocities and directions of the seven stars allowed this determination to be made. Measurements revealed a central mass equal to 8,200 suns, while visual surveys of the region revealed no objects resembling stars. This region, which the team determined to be “light-months” wide, is what would be expected if a black hole were located.
Our galaxy is mature enough to grow a supermassive black hole at its heart, far beyond the point where it would have many intermediate-mass black holes of its own. It’s in the Milky Way, the team says, because cannibalization of its original galaxy happened to slow down its growth processes.
“Previous studies have asked ‘So where are the superfast stars?’ “We now have the answer and confirm that Omega Centauri has an intermediate-mass black hole,” Heberle said. “At about 18,000 light-years away, this is the closest example of a supermassive black hole.”
Of course, this doesn’t actually change Sgr A*’s status as the closest supermassive black hole to Earth, or Gaia BH1’s status as the closest stellar mass black hole to Earth—but it does give some reassurance that scientists are on the right track. Follow along as we consider how our central black hole became such a cosmic titan in the first place.
The team’s study was published Wednesday (July 10) in the journal Nature.
