At the heart of almost every large galaxy lies an object of immense proportions — a supermassive black hole. Up to billions of times more massive than our sun, these titans drive the evolution of the galaxies they inhabit.
Yet astronomers can’t figure out how they got so big. Some appear to have formed as early as 600 million years after the Big Bang, when the universe was just 4% of its current age. From our understanding of black hole growth, that seems impossible. “There is simply not enough time to build such a massive black hole so early in the universe,” said Łukasz Wyrzykowski, an astronomer at Warsaw University. Without, that is, something to seed their growth, he said.
Those “seeds” are believed to be intermediate-mass black holes — giant black holes that fall in a mass gap between stellar-mass black holes, formed from dead stars, and supermassive black holes. Intermediate-mass black holes should weigh anywhere from 100 to 100,000 solar masses, and they’re thought to form a crucial step in the growth of the monsters at the centers of galaxies.
The main problem is locating them. “Black holes don’t emit anything,” said Daniel Holz, an astrophysicist at the University of Chicago. “So they’re just really hard to find.”
Astronomers have already identified a few potential intermediate-mass black hole candidates. Last year, they used the Hubble Space Telescope to catch what may be a 50,000-solar-mass black hole eating a star; another 20,000-solar-mass candidate, HLX-1, may be doing the same.
Now researchers say that they’ve used an entirely new method to find a black hole that’s up to 55,000 solar masses. The discovery, published today in Nature Astronomy, introduces a search strategy that has the potential to find many more candidates in the future.
The research was led by James Paynter, a doctoral student at the University of Melbourne. In 2018, Paynter’s supervisor and co-author Rachel Webster asked him to look through a data set of about 2,700 gamma-ray bursts — bright explosions of energy thought to originate from merging neutron stars or giant supernovas — gathered by NASA’s Compton Gamma Ray Observatory between 1991 and 2000.
He was looking for instances where two nearly identical gamma-ray bursts appeared in quick succession. The double flash could indicate that a gamma-ray burst was being “lensed” by an object between it and us — a massive object bending the light of the explosion as it made its way to Earth. A massive object, perhaps, like an intermediate-mass black hole.
In the entire data set of 2,700 gamma-ray bursts, Paynter’s automated software flagged just one event. In 1995, Compton saw a flash from a suspected gamma-ray burst that went off when the universe was about 3 billion years old. Half a second later, it saw an almost identical burst.
The team concluded that an intermediate-mass black hole sat between us and the gamma-ray burst. The gamma-ray burst was slightly offset from the center of the black hole, so its light took two paths, one slightly longer than the other. “The lens affects the path for two photons going around opposite ends,” said co-author Eric Thrane, an astrophysicist at Monash University. “That’s the delay time.”