Black holes are some of the strangest objects in the universe, and they typically fall into one of two size extremes: "small" ones that are dozens of times more massive than the sun and other "supermassive" black holes that are billions of times larger than our nearest star. But until now, astronomers had not seen good evidence of anything in between.
A recent discovery of an intermediate-mass black hole in the nearby galaxy Messier 82 (M82) offers the best evidence yet that a class of medium-size black holes exists. The finding may provide a missing link that could explain how supermassive black holes — which are found at the centers of most, if not all, galaxies — come to be, researchers say.
"We know that supermassive black holes exist at the centers of almost every massive galaxy, but we don't know how [they] form," said Dheeraj Pasham, an astronomy graduate student at the University of Maryland, College Park, who led the research. [ The 9 Biggest Unsolved Mysteries in Physics ]
Insatiable giants
A black hole is a region of space where the gravitational field is so strong that neither matter nor light can escape. Though it can't be seen directly, astronomers can infer a black hole's existence by the way its gravity tugs on nearby matter, and from the radiation it spews out as bits of material falling into the black hole rub against one another, producing friction.
Astronomers have detected stellar-mass black holes, which are 10 to 100 times the mass of the sun, and supermassive black holes, which are hundreds of thousands to billions of solar masses. But the intermediate-mass variety has proved very difficult to detect, causing some to doubt their existence.
The recently identified medium-size specimen has a mass about 400 times that of the sun (give or take 100), according to the study published Sunday (Aug. 17) in the journal Nature. Scientists had hypothesized that such intermediate black holes existed, but this is the first time that one has been measured so precisely, the researchers said.
Astronomers know how stellar-mass black holes form: A massive star collapses under its own gravity. But such a process would seem unable to explain how much larger black holes arise, because they can only gobble material up to a rate known as the Eddington limit, and the universe isn't old enough for them to have grown from stellar mass to supermassive, said Cole Miller, an astronomer also at the University of Maryland.
"If you feed matter to the black hole too fast, it produces so much radiation that it blows away the matter that's trying to [accumulate]," Miller told Live Science.
Building a black hole
How, then, might supermassive black holes form? Some theories suggest these strange behemoths grew from intermediate-mass black holes — which act as "seeds" — that formed in the early stages of the universe from the collapse of giant clouds of gas.
Others say these black hole giants started out as stellar-mass black holes that somehow gobbled up material at a rate much faster than the typical limit.
Miller has theorized that maybe a dense cluster of stars merged in the early universe, "colliding with each other and sticking together like wet clay," producing a black hole that gathers mass at a rate exceeding the normal limit. "If you can evade that limit, you might be able to build bigger black holes," he said.
Priyamvada Natarajan, a theoretical physicist at Yale University in New Haven, Connecticut, and her colleagues recently developed a new theoretical concept that suggests it is possible to grow black holes from a stellar mass seed faster than the Eddington limit, if the seed is trapped in a star cluster feeding off cold, flowing gas. The research was detailed Aug. 7 in the journal Science.
The finding of an intermediate-mass black hole in a nearby galaxy is exciting because it provides a "missing piece" between stellar-mass black holes and supermassive ones, Natarajan told Live Science.
"We have very young black holes that are like the infant stage, and we have geriatric ones," Natarajan told Live Science. Intermediate mass black holes are like the teenagers, she said.
Now that Pasham's team has shown that at least one of these adolescent black holes exists, astronomers will no doubt look for more.
"There's very exciting science here," Natarajan said. "The discovery space is wide open."
This article was updated at 7:37 p.m. ET Aug. 19. Dr. Natarajan's developed a new theoretical concept for growing black holes rapidly, not a computer model.
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