The largest black hole ever detected is 36 billion times the mass of our Sun. It exists near the upper limit predicted by our cosmological models, leaving astronomers with burning questions surrounding the relationship between black holes and their galaxy hosts.
In a paper published August 7 in Monthly Notices of the Royal Astronomical Society, researchers announced the discovery of a black hole inside a supermassive galaxy 5 billion light-years from Earth, dubbed the Cosmic Horseshoe. The newly spotted monster is roughly 10,000 times heavier than the supermassive black hole at the Milky Way’s core, according to a statement. Theoretical predictions set the upper bound of a black hole’s mass at 40 to 50 billion times that of the Sun; this cosmic behemoth stands at 36 billion times the Sun’s mass, so it comes precariously close to what calculations allow.
The Cosmic Horseshoe’s enormous size visibly warps spacetime, bending the light from nearby galaxies into a horseshoe-shaped glare called an Einstein Ring. This fortuitous celestial quirk, along with more traditional detection methods, allowed astronomers to spot the new black hole, which has yet to be named.
“This is amongst the top 10 most massive black holes ever discovered, and quite possibly the most massive,” Thomas Collett, study co-author and a cosmologist at the University of Portsmouth in England, said in the statement. “Most of the other black hole mass measurements are indirect and have quite large uncertainties, so we really don’t know for sure which is biggest.”
Most large galaxies appear to host supermassive black holes at their core, including the Milky Way. Cosmological models predicted that bigger galaxies, like the Cosmic Horseshoe, might be capable of hosting even larger, “ultramassive” black holes. However, such ultramassive black holes were difficult to spot, as the conventional method of tracking the motion of stars around them—stellar kinematics—wasn’t effective for dormant, faraway black holes.
The researchers overcame this limitation with gravitational lensing, a method that doesn’t depend on necessarily “seeing” the motion of cosmic entities. They also took observational data from the Very Large Telescope and the Hubble Space Telescope to create a comprehensive model of the galaxy. This two-pronged approach allowed the team to spot a “dormant” black hole “purely on its immense gravitational pull and the effect it has on its surroundings,” explained Carlos Melo, study lead author and PhD student at the Universidade Federal do Rio Grande do Sul in Brazil, in the same statement.
“We detected the effect of the black hole in two ways,” Collett said. “It is altering the path that light takes as it travels past the black hole, and it is causing the stars in the inner regions of its host galaxy to move extremely quickly. By combining these two measurements, we can be completely confident that the black hole is real.”
“What is particularly exciting is that this method allows us to detect and measure the mass of these hidden ultramassive black holes across the universe,” Melos added, “even when they are completely silent.”
Another notable aspect of the Cosmic Horseshoe’s environment is that it’s a “fossil group.” These dark, massive systems are primarily driven by gravitational forces and usually come as the final product of a series of galaxy mergers.
“It is likely that all of the supermassive black holes that were originally in the companion galaxies have also now merged to form the ultramassive black hole that we have detected,” said Collett. “So we’re seeing the end state of galaxy formation and the end state of black hole formation.”
The new black hole is clearly impressive, and it’ll be exciting to see what else astronomers discover about it. It’s also a fantastic demonstration of multi-messenger astronomy—the coordination of different signal types from the same astronomical event. This has been essential in redefining phenomena that we supposedly “finished” studying, but it’s promising to see it support entirely new discoveries. Either way, there’s no doubt that we’re inching closer than ever to the core of our universe’s many mysteries.
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