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 connection 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 less massive than aptly named supermassive blacks, which have masses from millions to billions of that of the sun. The closest stellar-mass black hole scientists have found is called Gaia-BH1, and it’s only 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, 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 remnant of an ancient galaxy cannibalized by our galaxy.
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This would suggest 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 had never occurred, this intermediate black hole may have grown to supermassive status as the Milky Way supermassive black hole Sagittarius A* (Sgr A*), which has a mass of 4.3 million times larger than that of the sun and located 27,000 light years from Earth.
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 from the collapse of stars 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.
Therefore, scientists suggest that supermassive black holes are born and grow due to the merging of chains of increasingly large black holes. This has been proven by the discovery of ripples in space-time, called gravitational waves, emanating from black hole mergers.
This process of merging and accreting black holes, combined with the large mass gap between stellar-mass black holes and supermassive black holes, means that there must be a population of intermediate-sized black holes.
However, these intermediate-mass black holes with masses between a few hundred and a few thousand times that of the Sun seem to have largely eluded detection. That’s because, like all black holes, these mid-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 material to feed on, which glows as it heats up, or they rupture and feed on an ill-fated star in a so-called “Tidal Disruption 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 hunting for such black holes. They use the gravitational effects these voids have on matter, such as stars orbiting them or on light passing through them. The team of this new discovery used the previous method.
A speed 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 .
In particular, researchers and collaborator Maximilian Häberle, Ph.D. student, wanted to find fast-moving stars in Omega Centauri that would prove the star cluster has a massive, dense, or compact “central engine” black hole. A similar method was used to determine the mass and magnitude of Sgr A* using a population of fast-moving stars at the heart of the Milky Way.
Häberle and team used over 500 Hubble images of this star cluster to build an extensive 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 for 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,” Häberle said. The team ultimately did not find a but seven “needle in a haystack” stars, all moving at high speed in a small region at the heart of Omega Centauri.
The rapid speed of these stars is caused by a mass concentrated 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, narrow central mass or whether that star is a runaway moving at a rapid rate in the a straight path – in the absence of any central measure.
Knowing and measuring the different speeds and directions of the seven stars allowed this determination to be made. The measurements revealed a centralized 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 located in this region, which the team determined to be “light-months” wide.
The fact that our galaxy has matured enough to have grown a supermassive black hole at its heart means that it has probably outgrown the stage of having many intermediate-mass black holes. This exists in the Milky Way, the team says, because cannibalization of its parent galaxy occurred to limit its growth processes.
“Previous studies had prompted the critical question, ‘So where are the high-velocity stars?’ We now have an answer to that and confirmation that Omega Centauri contains an intermediate-mass black hole,” Häberle said. about 18,000 light-years away, this 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 provide some reassurance that scientists are on it right track when you 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.