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 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 around 8,200 suns, which makes it considerably 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, which have mass millions to billions that of the sun. The closest stellar-mass black hole scientists have found is called Gaia-BH1, and it sits only 1,560 light-years away from us.
The newly found intermediate-mass black hole, on the other hand, dwells in a spectacular collection of about ten million stars called Omega Centauri, which sits around 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 cannibalized by our own galaxy.
Related: Watch a supermassive black hole trap a 'fluffy' disk in this simulation
This would suggest Omega Centauri is actually the core of a small, separate galaxy whose evolution was cut short when the Milky Way swallowed it. If this event had never happened, this intermediate black hole may have possibly grown to supermassive status like the Milky Way's own supermassive black hole, Sagittarius A* (Sgr A*), which has a mass 4.3 million times that of the sun and is located is 27,000 light-years from Earth.
Hunting for what's missing
Scientists have known for some time that not all black holes are created equally. While stellar-mass black holes are known to form via the collapse of 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 of times as massive as the sun.
Therefore, scientists propose that supermassive black holes are born and grow due to merger chains of progressively larger and larger black holes. This has been evidenced by the detection of ripples in spacetime, called gravitational waves, emanating from black hole mergers.
This process of black hole mergers and growth, combined with the vast gap in mass between stellar-mass black holes and supermassive black holes, means there should be a population of mid-size black holes.
Yet, these intermediate-mass black holes with masses between a few hundred and a few thousand times that of the sun have, for the most part, seem to have avoided 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 immense that not even light is 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 while heating up, or rip apart and feed on an unfortunate star in a so-called "Tidal Disruption Event" (TDE).
Intermediate black holes, like the one in Omega Centauri, aren't surrounded by a lot of matter and feeding.
That means astronomers have to be a little bit cunning when hunting for such black holes. They use the gravitational effects these voids have on matter, like stars that orbit them or on light passing through them. This new discovery's team used the former method.
A speeding 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 Omega Centauri's formation history.
In particular, the researchers, and collaborator Maximilian Häberle, an MPIA Ph.D. student, wanted to find rapidly 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 size of Sgr A* using a fast-moving population of stars at the heart of the Milky Way.
Häberle and team used over 500 Hubble images of this star cluster to build a vast database of the motions of stars in Omega Centauri, measuring the speeds of about 1.4 million stars. This ever-repeating view of Omega Centauri, which Hubble conducted not out of scientific interest but rather to calibrate its instruments, was the ideal data set for the team's mission.
"Looking for high-speed stars and documenting their motion was the proverbial search for a needle in a haystack," Häberle said. The team ultimately found not one but seven "needle-in-haystack stars," all moving at rapid velocities in a small region at the heart of Omega Centauri.
The rapid speed of these stars is caused by a concentrated mass nearby. If the team had only found one rapid star, it would have been impossible to determine whether its speed was the result of a large and close central mass or if that star is a runaway moving at a rapid pace in a straight path — absent of any central mass.
Spotting and measuring the different velocities and directions of 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 objects that resemble stars. That is exactly what would be expected if a black hole was 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 it has probably outgrown the stage of possessing many intermediate-mass black holes of its own. This one exists in the Milky Way, the team says, because the cannibalization of its original galaxy happened to curtail its growth processes.
"Previous studies had prompted critical questions of 'So where are the high-speed stars?' We now have an answer to that and the confirmation that Omega Centauri contains an intermediate-mass black hole," Häberle said. "At a distance of about 18,000 light-years, this is the closest known example of a massive black hole."
Of course, that doesn't really change the status of Sgr A* as the closest supermassive black hole to Earth, or Gaia BH1's status of the closest stellar-mass black hole to Earth — but it provides some reassurance that scientists are on the right track when considering 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.
