by Using the Hubble Space Telescope, astronomers have discovered the closest supermassive black hole to Earth ever seen, a cosmic Titan “frozen in time.”
As an example of an “unacceptably medium-sized black hole,” the object could serve as a missing link in understanding the connection between stellar mass and supermassive black holes. The black hole appears to have a mass of about 8,200 suns, making it much more massive than stellar mass black holes with masses between 5 and 100 times that of the sun, and much less massive than aptly named supermassive black holes, with masses millions to billions of those of the sun. The closest stellar mass black hole scientists have found is called Gaia-BH1, and it’s located just 1,560 light-years away.
On the other hand, the newly discovered intermediate mass black hole resides in a stunning collection of about ten million stars called Omega Centauri, which sits 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 remains of an ancient galaxy that has been cannibalized by our own galaxy.
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This would suggest that Omega Centauri is at the heart of a separate small galaxy whose evolution was cut short when it was swallowed up by the Milky Way. If this event had never occurred, this intermediate black hole may have grown to supermassive status similar to the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*), which has a mass of 4.3 million times more than the sun and is located. which is 27,000 light years from Earth.
Hunt for what is missing
Scientists have known for some time that not all black holes are created equal. Although supermassive black holes are known to form stars from collapsed stars with at least eight times the mass of the Sun, supermassive black holes must have a different origin. That’s because no star is massive enough to collapse and leave a remnant millions sometimes as huge as the sun.
Therefore, scientists suggest that supermassive black holes are born and grow due to merging chains of gradually larger and larger black holes. This is evident when ripples in space-time, known as gravitational waves, arising from black hole mergers have been detected.
This process of black hole mergers and growth, combined with the huge gap in mass between stellar-mass black holes and supermassive black holes, means that there should be a population of medium-sized black holes.
However, these medium-sized black holes with masses ranging from a few hundred to a few thousand times that of the sun have, for the most part, eluded 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 massive that even light is not fast enough to escape it. Therefore, black holes only show up in the light if they are surrounded by material to feed on, which burns and heats up, or rips apart and feeds an unlucky star in a “Tidal Disruption Event” ( TDE) as it is called.
Interstellar black holes, like the one in Omega Centauri, are not surrounded by much matter and nourishment.
That means astronomers have to be a little cautious when hunting for such black holes. They use the gravitational effects these voids have on matter, such as stars orbiting them or the light passing through them. The team of this new discovery used the former method.
A star in motion
The pursuit of this intermediate black hole began in 2019 when Nadine Neumayer from the Max Planck Institute for Astronomy (MPIA), and Anil Seth from the University of Utah designed a research project to improve our understanding of the formation history of Omega Centauri.
In particular, the researchers, and collaborator Maximilian Häberle, MPIA Ph.D. student, trying to find fast-moving stars in Omega Centauri that would indicate that the star cluster has a massive, compact black hole or “central engine”. A similar method was used to find the mass and size of Scribe A* using the fast population of stars in the Milky Way’s core.
Häberle and his 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 speeds of about 1.4 million stars. This repeated view of Omega Centauri, made by Hubble not out of scientific interest but to calibrate its instruments, was the ideal data set for the team’s mission.
“Searching for high-velocity stars and documenting their motion was the proverbial search for a needle in a haystack,” said Häberle. The team only got one in the end seven “needle-in-a-haystack stars,” all moving at a fast pace in a small region in the heart of Omega Centauri.
The fast speed of these stars is caused by a nearby concentrated mass. If the team only found one fast star, it would be impossible to determine whether its speed was the result of a large and close central mass or whether that star was a star moving at fast speed in a straight path — no central mass.
By observing and measuring the different velocities and directions of seven stars, this determination was made. The measurements showed a centralized mass equal to 8,200 suns, and visual inspections of the region revealed no star-like objects. That’s exactly what would be expected if a black hole were located in this region, which the team determined would be “light months” across.
Since our galaxy has matured enough to have a supermassive black hole at its core, it has likely grown beyond the state of having many medium-sized black holes of its own. This one exists in the Milky Way, the team says, because the cannibalization of its original galaxy occurred to limit its growth process.
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“Previous studies have raised critical questions about ‘Where are the high-velocity stars?’ We now have an answer to that and the confirmation that Omega Centauri has a medium-sized black hole,” said Häberle. “About 18,000 light-years away, this is the closest known example of a supermassive black hole.”
Of course, that doesn’t really 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 provide some reassurance that the scientists on the right. Track as you wonder how our central black hole became such a Cosmic Titan in the first place.
The team’s research was published on Wednesday (July 10) in the journal Nature.
