HONOLULU — Perhaps most supermassive black holes — dark giants in the centers of galaxies — are just shy when they’re young.
“We have this weird problem, where on the one hand the universe makes really supermassive black holes very shortly after the Big Bang,” says Kevin Schawinski, an astrophysicist at ETH Zürich in Switzerland. “But when we look at more typical galaxies, we find no evidence for growing black holes.”
The feeding zones around voracious black holes create quasars, blazing furnaces of X-rays and other light. And yet the Chandra space telescope detects no X-rays from a cache of galaxies in the constellation Fornax that researchers think should be nourishing young black holes, Schawinski reported August 6 at a meeting of the International Astronomical Union.
Over the past several years, astronomers have found a handful of very bright quasars that lit up within the first billion years of cosmic history. These quasars are probably powered by unusually hefty supermassive black holes — ones that gobbled down gas as fast as physically possible (or even faster) for hundreds of millions of years.
“If this happens all over the universe,” says Schawinski, “then if we look at more normal-mass galaxies, we should be seeing their supermassive black holes pop out in the early universe to the same degree.”
But they don’t.
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Maybe the more run-of-the-mill black holes are there but they’re not actively feeding, he says. Or perhaps something is blocking the X-rays from getting out.
Or maybe — just maybe — these black holes haven’t been born yet.
“It’s a very interesting suggestion,” says Andrea Comastri, an astronomer at the Osservatorio Astronomico di Bologna in Italy, says of the not-yet-born scenario. “But I’m not convinced.”
These images capture a relatively tiny volume of space, he says, so perhaps the researchers aren’t casting a wide enough net. The distances to these galaxies are also notoriously difficult to pin down. Many could be much closer and seen during a time when black holes have formed but quieted down a bit.
If the universe can make monstrous black holes in under a billion years, then making the relatively little guys should be straightforward and they should be everywhere, Comastri says. “It should be easier to make smaller black holes because you don’t have to work that much. They are there somewhere.”
If the black holes are confirmed to be missing, “it’s going to shake up a lot of what we think about the growth of quasars,” says Tiziana Di Matteo, an astrophysicist at Carnegie Mellon University in Pittsburgh. “But I’m very skeptical of it.”
These cosmic no-shows probably don’t suck down gas as fast as the researchers assume, she says. If these black holes only nibble at the surrounding gas — as opposed to their obese cousins who gorge themselves — then X-rays would only trickle from their dinner plates and might not be detected.
Much like with humans, black hole obesity is influenced by environment. Most galaxies need some time to build up enough mass to efficiently feed their black holes, Di Matteo says. Tiny galaxies easily lose gas every time a cluster of new stars is born or whenever a dying star explodes. “It’s only in extreme environments,” she says, at the junctions of cosmic filaments that become interstellar dumping grounds, “where gas could plunge through and not care about anything else that’s going on.” Here, fledgling black holes aren’t as reliant on their galaxy’s feeble gravity to grab food; the incoming rivers of gas are like intergalactic fire hoses.
Those unusually massive black hole starter kits are probably responsible for the dazzling quasars that switch on during the first billion years after the Big Bang. Computer simulations show that in the younger, more intimate universe, when everything was squished together a lot more than today, there are the oddball places where gas funnels onto ancestral galaxies at astounding rates, providing fast-growing black holes with an all-you-can-eat buffet.
The other less showy black holes, the ones Schawinski and colleagues are hunting for, probably spend the next several billion years quietly catching up. Finding these black holes when they’re young and struggling to grow might require searching a wider area or getting more sensitive observations.
“It’s exciting,” Schawinski says. “It’s the last major category of astrophysical objects of whose origin we know nothing about.” Planets, stars and galaxies are pretty well understood, he says. “But we have no idea how supermassive black holes form.”
Schawinski’s team plans to spend the next year or two repeating their experiment over a wider volume of space, hoping to find at least one youthful black hole in a moderate-sized galaxy. “Once you go from zero to one you have something to work with,” he says. “Right now we’ve got nothing.”