Using the Hubble Space Telescope, astronomers have discovered the closest massive black hole to Earth ever observed, a cosmic titan “frozen in time.”
As 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 a mass of about 8,200 suns, making it considerably more massive than stellar-mass black holes that have masses between 5 and 100 times that of the sun, and much less massive than supermassive black holes, which have masses of millions to billions of the sun. The closest stellar-mass black hole that scientists have discovered is called Gaia-BH1, and it is just 1,560 light-years away.
The newly discovered intermediate-mass black hole is located in a spectacular cluster of about ten million stars called Omega Centauri, located about 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 suggests that Omega Centauri is actually the core of a separate, small galaxy whose evolution was interrupted when the Milky Way swallowed it. If that event had never occurred, this intermediary black hole could have attained supermassive status like the Milky Way’s supermassive black hole, Sagittarius A* (Sgr A*), which has a mass 4.3 million times that of the Sun and is 27,000 light-years from Earth.
In search of 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 from the collapse of stars that are 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 times more massive than the sun.
Scientists therefore propose that supermassive black holes are born and grow as a result of the merger of chains of increasingly larger black holes. This has been demonstrated by the detection of ripples in space-time, called gravitational waves, emanating from the merger of black holes.
This process of black hole merger and growth, combined with the huge mass gap between stellar-mass black holes and supermassive black holes, means that there should be a population of medium-sized black holes.
Yet these intermediate-mass black holes, whose masses range from a few hundred to a few thousand times that of the Sun, seem to have mostly escaped detection. Indeed, like all black holes, these medium-sized cosmic titans are marked by outer boundaries called event horizons.
The event horizon is the point where a black hole’s gravitational pull becomes so immense that even light isn’t fast enough to escape it. So black holes are only visible to light if they’re surrounded by material they feed on, which glows as it heats up, or if they tear themselves apart and feed on an unfortunate star in what’s called a “tidal disruption event” (TDE).
Intermediate black holes, like the one in Omega Centauri, are not surrounded by much matter and food.
That means astronomers have to get a little clever to look for such black holes. They use the gravitational effects that these voids have on matter, such as the stars that orbit them, or on the light that passes through them. The team behind this new discovery used the first method.
A shooting star
The hunt for this intermediate black hole began in 2019 when Nadine Neumayer 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 the formation history of Omega Centauri.
The researchers and their collaborator Maximilian Häberle, a doctoral student at MPIA, wanted in particular to find fast-moving stars in Omega Centauri, which would prove that the star cluster has a massive, dense or compact “central driving” black hole. A similar method was used to determine the mass and size of Sgr A* using a population of fast-moving stars in the heart of the Milky Way.
Häberle and his team used more than 500 Hubble images of the star cluster to build a vast database of Omega Centauri’s star motions, measuring the velocities of about 1.4 million stars. This repeat view of Omega Centauri, which Hubble made not for scientific interest but rather to calibrate its instruments, was the ideal dataset for the team’s mission.
“Looking for high-speed stars and documenting their motion was like looking for a needle in a haystack,” Häberle said. The team ultimately found not one, but Seven “stars like needles in a haystack,” all moving at rapid speeds in a small region at the heart of Omega Centauri.
The fast speed of these stars is due to a concentrated mass nearby. If the team had found only one fast star, it would have been impossible to determine whether its speed was the result of a large, nearby central mass or whether this star is a runaway star moving at high speed on a straight path, with no central mass.
Detecting and measuring the different speeds and directions of seven stars helped determine this hypothesis. The measurements revealed a central mass equivalent to 8,200 suns, while visual inspections of the region revealed no star-like objects. This is exactly what would be expected if a black hole were in this region, which the team determined to be “light-months” wide.
The fact that our galaxy is mature enough to have developed a supermassive black hole at its core means that it has probably moved beyond the stage where it has many intermediate-mass black holes of its own. This one exists in the Milky Way, the team explains, because the cannibalization of its parent galaxy has limited its growth process.
“Previous studies have raised crucial questions such as: ‘Where are the high-speed stars?’ We now have an answer to this question and confirmation that Omega Centauri contains an intermediate-mass black hole,” Häberle said. “At a distance of about 18,000 light-years, it is the closest known example of a massive black hole.”
Of course, this 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 reassure scientists that they’re on the right track when examining 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.