Science News has a metaphor for our times playing out in deep space, where we’ve been watching black holes defy expectations by getting much bigger, much faster than we figured:
Sightings since 2006 have shown that gargantuan monsters with masses of at least a billion suns were already in place when the universe was less than a billion years old – far too early for them to have formed by conventional means.
One or two of these old massive objects could be dismissed as freaks, says theoretical astrophysicist Priyamvada Natarajan of Yale University. But to date, astronomers have spotted more than 100 supermassive black holes that existed before the universe was 950 million years old. “They’re too numerous to be freaks now,” she says. “You have to have a natural explanation for how these things came to be.”
Astronomers express how fast a black hole is eating with a term called the Eddington ratio, measuring the black hole’s actual brightness in relation to the brightness it would have if it were eating as fast as it possibly could.
A 100-solar-mass black hole accreting at the limit should take about 800 million years to reach a billion solar masses, even taking into account that it would eat faster as it grew. And that 800 million years doesn’t include the time it took the initial black hole seed to form.
But physicist Myungshin Im of Seoul National University in South Korea and colleagues worried that previous observations were missing pickier eaters because fast eaters are brighter and easier to spot. If some early massive black holes were lazy eaters, their super sizes become even more puzzling — and may rule out some theories for how they grew.
The researchers found IMS J2204+0112, a billion-solar-mass black hole eating at a tenth of its speed limit and hailing from when the universe was about 940 million years old. But at its feeding rate, the black hole shouldn’t have fully matured until the universe was 8 billion years old, the team reported on arXiv.org February 9.
“We show for the first time that quasars with low Eddington ratio exist in the early universe,” Im says.
IMS J2204+0112 is the dimmest slow-eating quasar spotted yet, but it’s not alone. Physicist Chiara Mazzucchelli of the Max Planck Institute for Astronomy in Germany and colleagues reported 11 fussy supermassive black holes that existed when the universe was less than 800 million years old, in the Astrophysical Journal last November.
On average, those quasars weigh in at around 1.62 billion solar masses but eat at about 40 percent of the speed limit, the team reported. Strangely, the largest black hole in that group, HSC J1205-0000, had the lowest feeding rate: The black hole is 4.7 billion solar masses yet eats at only 6 percent of its limit.
It was strange enough to find supermassive black holes with gluttonous appetites in the early universe, but these picky eaters are even harder to explain.
Astronomers hope peering farther back in time will help find the “seed” black holes that may grow into behemoths. If some black holes started out huge, from 10,000 to a million solar masses, they could grow even larger either by merging with each other or accreting at the Eddington limit.
“If you start with such a very massive seed, you have a jump-start,” says astrophysicist Avi Loeb of Harvard University. “Then you don’t need as much time to grow to a billion solar masses.”