Using the Hubble Space Telescope, astronomers have discovered the closest supermassive black hole ever seen to Earth, a cosmic titan “frozen in time.”
As an elusive example of an “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 about 8,200 solar masses, making it significantly more massive than stellar-mass black holes, which have masses between 5 and 100 times that of the sun, and much less massive than properly supermassive black holes, which have masses ranging from millions to billions of solar masses. The nearest stellar-mass black hole scientists have found is called Gaia-BH1, and it’s just 1,560 light-years away.
The newly found intermediate-mass black hole is located within a magnificent stellar 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 has stopped growing supports the idea that Omega Centauri is the remnant of a former galaxy that was torn apart by our own.
Relating to: Watch how a supermassive black hole traps a ‘hairy’ disk in this simulation
This suggests that Omega Centauri was actually the nucleus of a small, separate galaxy whose evolution was cut short by the Milky Way’s engulfment. If this event had never occurred, this intermediate black hole would likely have attained supermassive status like the Milky Way’s own supermassive black hole, Sagittarius A* (Sgr A*). Sagittarius A* is 4.3 million times the mass of the Sun and is 27,000 light-years from Earth.
Searching for the lost
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 with a mass of at least eight times the mass of the sun, supermassive black holes must have a different origin, as no star is massive enough to collapse and leave a remnant. Millions As big as the sun.
Therefore, scientists suggest that supermassive black holes are born and grow due to chains of mergers of larger and larger black holes. This has been proven by detecting ripples in space-time, called gravitational waves, emitted from black hole mergers.
The process of black hole merger and growth, combined with the enormous mass difference between stellar-mass black holes and supermassive black holes, suggests that a population of intermediate-sized black holes should form.
Yet these intermediate-mass black holes, with masses between a few hundred and a few thousand times that of the sun, appear to have mostly escaped 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 pull of a black hole becomes so great that even light cannot escape it. Therefore, black holes can only be seen in light if they are surrounded by matter that they can feed on, and glow as they heat up, or if they break apart and feed on an unfortunate star in a so-called “Tidal Disruption Event” (TDE).
Intermediate black holes like the one at Omega Centauri are not surrounded by much matter and nutrients.
This means astronomers have to be a bit crafty when hunting for these types of black holes. They use the gravitational effects of these voids on matter, such as stars orbiting them or light passing through them. The team behind this new discovery used the old method.
An accelerating 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.
Specifically, the researchers and their collaborator, MPIA doctoral student Maximilian Häberle, wanted to find rapidly moving stars in Omega Centauri that would prove that the star cluster has a large, dense or compact “central engine” black hole. A similar method was used to determine the mass and size of Sgr A* using a population of fast-moving stars at the heart of the Milky Way.
Häberle and his team used more than 500 Hubble images of the star cluster to create a large database of the motions of stars in Omega Centauri, measuring the speeds of about 1.4 million stars. This continuously repeated imaging of Omega Centauri, which Hubble took not for scientific interest but to calibrate its instruments, was the ideal dataset for the team’s mission.
“Looking for high-velocity stars and documenting their motion was like finding a needle in a haystack,” Häberle said. The team ended up finding not one, Seven “Needle in a Haystack Stars” moving at high speeds in a small region at the heart of Omega Centauri.
The fast speed of these stars is due to a dense 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 that star was a runaway star moving at a fast pace in a straight path without any central mass.
Detecting and measuring the different speeds and orientations of seven stars allowed this determination to be made. The measurements revealed a central mass equivalent to 8,200 suns, while visual inspections of the region revealed no objects resembling stars. This is what the team would expect if a black hole were to be found in this region, which they determined to be “light-moons” across.
The fact that our galaxy has matured enough to form a supermassive black hole at its centre means that it is likely past the stage of having many intermediate-mass black holes of its own. The team says that the reason it exists in the Milky Way is because the cannibalisation of its original galaxy has constrained its growth processes.
RELATED STORIES:
— Dark matter detected hanging from the cosmic web for the first time
— Exotic ‘Einstein ring’ suggests mysterious dark matter interacts with itself
— Tiny black holes left over from the Big Bang may be prime suspects for dark matter
“Previous studies raised critical questions like, ‘Where are the high-velocity stars?’ Now we have an answer and confirmation that Omega Centauri contains an intermediate-mass black hole,” Häberle said. “At about 18,000 light-years away, it’s the closest known example of a massive black hole.”
Of course, this doesn’t 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 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 in the journal Nature on Wednesday (July 10).