When black holes collide, the resulting cosmic drama was supposed to unfold under the cover of darkness, since both objects are invisible. But now astronomers believe they have made the first optical observations of such a fusion, marked by a glow of light a billion times brighter than the sun.
The eruption was related to a known black hole fusion detected last year by the gravitational wave observatory, Ligo, which picked up waves sent through the structure of space. The latest observations suggest that when these cataclysmic events occur within the accretion disk of an even more gigantic black hole, they are brilliantly illuminated by the surrounding dust and gas, making them also visible to optical telescopes.
“This supermassive black hole had been bubbling for years before this most abrupt flare,” said Matthew Graham, professor of astronomy research at the California Institute of Technology and lead author of the paper. “We concluded that the eruption is likely the result of a merger of black holes.”
The authors have not entirely ruled out other sources, but Saavik Ford, a co-author at New York City University, said the window of doubt was narrow. “We are 99.9% safe,” he said.
Professor Alberto Vecchio, director of the University of Birmingham Gravitational Wave Astronomy Institute, said the experts would now be watching closely to see how the latest observations align with a detailed analysis of the same event to be released in the coming months. by Ligo scientists. “If the two independent observations align … this would really be quite a spectacular thing,” he said.
The observations came after Ford and his colleague, Barry McKernan, made theoretical predictions that black hole mergers would be visible, contrary to expectations, if they occurred in the context of the accretion disk of a third supermassive black hole.
Ford and McKernan teamed up with Graham, a project scientist for the Zwicky Transitional Facility (ZTF), a study telescope designed to detect bright events. “It’s perfect for something like this,” said Ford.
Scientists tracked Zwicky’s data for any flares that matched the time and place with the known collisions Ligo had detected, which releases public alerts every time a detection is made. One event stood out: a merger known as S190521g that Ligo detected in May of last year.
“It certainly is not one of the things that I would have predicted three years ago when we started the survey,” Graham said.
Further analysis suggested that the merger had taken place near a distant supermassive black hole called J1249 + 3449, with a diameter equivalent to Earth’s orbit around the sun. The smallest pair of black holes sat on the outer reaches of the accretion disk, a halo of stars, dust, and gas swirling around the large central sump. “These objects swarm like angry bees around the monstrous queen bee in the center,” said Ford.
As the pair of black holes, each the size of the Isle of Wight and with a combined mass of 150 suns, spiral inward and come together, gravitational waves are sent through space and the new combined object experiences a kick in the opposite direction, sending it through the dust and gas of the disc and into the surrounding space.
“It is the reaction of the gas to this bullet that creates a bright flare, visible with telescopes,” said McKernan.
If confirmed, the observations could help solve a central problem in black hole astronomy: that black holes are much heavier than they should be. Black holes are formed from collapsed ancient stars. The largest black holes form when they merge, but some black holes are so large that, in theory, it should have taken longer than the age of the universe for them to shoot up to the observed size.
One possible explanation is that if black holes are grouped into accretion disks, then there are likely to be multiple rounds of mergers. “If you have a place where you can hold these black holes in one place, you can match them efficiently,” Vecchio said. “Gas is the glue that holds them together.”
Ligo’s observations can’t easily solve this question, because gravitational-wave astronomy can’t determine exactly where in the sky a fusion has occurred, but if the same events could be seen using conventional telescopes, an answer might be imminent. The findings are published in the journal Physical Review Letters.
