Astronomers have discovered what could be the largest black hole ever detected. With a mass of 36 billion times that of our Sun, its gravity is so powerful that it bends the light of an entire galaxy behind it into a near-perfect circle called an Einstein ring, effectively reducing a realm with trillions of stars of its own into an astrophysical fashion accessory. It’s 10,000 times as heavy as our Milky Way’s own central black hole, and is nigh unto breaking the universe’s theoretical upper limit. If anything ever warranted being called a cosmic monster, it’s this.
“This is amongst the top ten most massive black holes ever discovered, and quite possibly the most massive,” Thomas Collett, a professor of astrophysics at the University of Portsmouth and coauthor of a new study about the giant in the journal Monthly Notices of the Royal Astronomical Society, said in a statement about the work. Other detections of similar sized objects, Collett noted, have generally come with uncertainties too large to be definitive.
This not-super but ultramassive black hole lurks in the center of the famous Cosmic Horseshoe galaxy, which itself ranks among the most massive ever spotted. The galaxy is considered a fossil group, which formed from other large galaxies — and their constituent supermassive black holes — collapsing together.
“So we’re seeing the end state of galaxy formation and the end state of black hole formation,” Collet said. It’s no exaggeration to say, then, that we’re literally witnessing a black hole’s final form.
Located some five billion light years away, the Cosmic Horseshoe is so named due to its gravitational lensing effect, a phenomenon in which the light of a background galaxy is warped by the gravity of a foreground one.
Lensing is common throughout the cosmos, and it can be a fortuitous tool for astronomers, acting like a magnifying glass that allows them to observe distant objects whose light would otherwise be too faint to examine. But in this case, the huge foreground galaxy and its companion in the background happen to be in almost perfect alignment with our Earthly perspective, bending the light into an incomplete ring.
Astronomers have long suspected that there was a black hole at the heart of the Cosmic Horseshoe, but have never been able to spot — let alone measure — it. One of the reasons why is its extreme distance, at billions of light years away. But the even more impressive hurdle that’s been overcome is that it’s a “dormant” black hole that’s no longer accreting matter, according to Carlos Melo, lead author from the Universidade Federal do Rio Grande do Sul in Brazil.
“Typically, for such remote systems, black hole mass measurements are only possible when the black hole is active,” Melo said. “But those accretion-based estimates often come with significant uncertainties.”
When a black hole devours significant amounts of matter, the infalling material gets heated up and radiates huge amounts of energy and light, forming what’s known as an active galactic nucleus. (The brightest of these are called quasars.) But this detection “relied purely on [the black hole’s] immense gravitational pull and the effect it has on its surroundings,” Melo said.
Their method involved a combination of lensing and what’s known as stellar kinematics, which allows astronomers to infer a black hole’s mass by studying the velocity of stars trapped in the surrounding galaxy.
“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, even when they are completely silent,” Melo said.
And its size is no coincidence. There’s a reason, the astronomers argue, that we’re finding this ultramassive rarity in one of the heaviest galaxies on record, and not in one of relatively unremarkable size like our Milky Way, which hosts a comparatively puny black hole of 4.3 million solar masses.
“We think the size of both is intimately linked,” Collet said, “because when galaxies grow, they can funnel matter down onto the central black hole.”
That may seem like an obvious conclusion to draw, but how supermassive black holes attain their enormous sizes remains one of the great mysteries of cosmology. Some have been spotted so early on in the universe’s history that they physically shouldn’t exist, not having enough time to accrete the mass they possess. If it formed from galactic mergers, it provides a strong clue of at least one mechanism that can spawn these colossal objects.
More on black holes: Bizarre “Infinity Galaxy” Could Hold the Secrets of Supermassive Black Holes
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