Using the Hubble Space Telescope, astronomers have discovered the closest massive black hole to Earth ever seen, a cosmic titan ‘frozen in time’.
As an example of an elusive “intermediate-mass black hole,” the object may serve as a missing link in understanding the relationship between stellar mass and supermassive black holes. The black hole appears to have a mass of about 8,200 suns, making it significantly more massive than stellar-mass black holes with masses between 5 and 100 times that of the sun, and much more massive than the aptly named supermassive black holes , which have a mass millions to billions of that of the sun. The closest stellar-mass black hole scientists have found is called Gaia-BH1, and it’s just 1,560 light-years away.
The newly discovered intermediate-mass black hole, on the other hand, resides in a spectacular collection of about ten million stars called Omega Centauri, which lies about 18,000 light-years from Earth.
Interestingly, the fact that the “frozen” black hole appears to have slowed its growth supports the idea that Omega Centauri is the remnant of an ancient galaxy that was cannibalized by our own galaxy.
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This suggests that Omega Centauri is actually the core of a small separate galaxy whose evolution was interrupted when the Milky Way swallowed it. If this event never happened, this intermediate black hole probably grew to supermassive status like the Milky Way’s own supermassive black hole, Sagittarius A* (Sgr A*), which is 4.3 million times more massive of the Sun and is located 27,000 light years from Earth.
On the hunt for what is missing
Scientists have known for some time that not all black holes are created equal. While stellar-mass black holes are known to form through the collapse of stars with at least eight times the mass of the Sun, supermassive black holes must have a different origin. This is because no star is massive enough to collapse and leave a remnant millions times more massive than the sun.
Therefore, scientists hypothesize that supermassive black holes are born and grow due to chains of mergers of progressively larger and larger black holes. This has been proven by the detection of ripples in space-time, called gravitational waves, emitted by merging black holes.
This process of black hole merger and growth, combined with the huge mass difference between stellar-mass black holes and supermassive black holes, means that there must be a population of intermediate-sized black holes.
Yet, these intermediate-mass black holes with masses between a few hundred and a few thousand times that of the Sun seem to have largely escaped detection. That’s because, like all black holes, these medium-sized cosmic titans are marked by outer boundaries called event horizons.
The event horizon is the point at which the gravitational influence of a black hole becomes so great that even light is not fast enough to escape it. Thus, black holes are only visible in light if they are either surrounded by matter to feed on, which glows as it heats up, or they tear apart and feed on an unfortunate star in a so-called “Tidal Event” (TDE).
Intermediate black holes, like the one in Omega Centauri, are not surrounded by much matter and food.
This means that astronomers have to be a little crafty when looking for such black holes. They exploit the gravitational effects these cavities have on matter, such as the stars orbiting them, or on the light passing through them. The team of this new discovery used the previous method.
Fast star
The search for this intermediate black hole began in 2019, when Nadine Neumeier of the Max Planck Institute for Astronomy (MPIA) and Anil Seth of the University of Utah developed a research project to improve our understanding of the formation history of Omega Centauri.
In particular, researchers and collaborator Maximilian Häberle, MPIA Ph.D. student, wanted to find fast-moving stars in Omega Centauri, which would prove that the star cluster has a massive, dense or compact black hole with a “central engine”. A similar method was used to determine the mass and size of Sgr A* using a fast-moving population of stars at the heart of the Milky Way.
Heberle and team used more than 500 Hubble images of this star cluster to build a huge database of the motions of the stars in Omega Centauri, measuring the velocities of about 1.4 million stars. This ever-repeating view of Omega Centauri, which Hubble made not out of scientific interest but rather to calibrate its instruments, was the perfect data set for the team’s mission.
“Searching for high-velocity stars and documenting their motion has been the proverbial search for a needle in a haystack,” Haberle said. The team eventually discovered more than one but seven “needle in a haystack stars” all moving at rapid speeds in a small region at the heart of Omega Centauri.
The fast speed of these stars is due to concentrated mass nearby. If the team had found just one fast star, it would have been impossible to determine whether its speed was the result of a large and nearby central mass, or whether that star was a runaway speeding along a straight path — lacking any central mass. .
Observing and measuring the different speeds and directions of seven stars allowed this determination to be made. The measurements reveal a centralized mass equivalent to 8,200 suns, while visual inspections of the region reveal no objects resembling stars. That’s exactly what would be expected if a black hole were located in this region, which the team described as “light months” wide.
The fact that our galaxy has matured enough to develop a supermassive black hole at its heart means that it has probably outgrown the stage of having many intermediate-mass black holes of its own. This exists in the Milky Way, the team says, because the cannibalization of its parent galaxy has limited its growth processes.
“Previous studies have raised critical questions, ‘Where are the high-velocity stars?’ example of a massive 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 confidence that scientists are tracking right when we consider how our central black hole became such a cosmic titan in the first place.
The team’s research was published Wednesday (July 10) in the journal Nature.
