New recipe found for making supermassive black hole

Take two primordial galaxies, slam them together to create ‘dark collapse’


ANCIENT HEAVYWEIGHT  A quasar like the one illustrated here is an ultrabright galaxy with a supermassive black hole at its center. Scientists want to know how black holes grew fast enough to fuel quasars found less than a billion years after the Big Bang.

M. Kornmesser/ESO

GENEVA — Monster black holes in the early universe may have taken an unusual route to becoming so massive.

Giant gas clouds in some of the universe’s first galaxies collapsed under their own gravity to form supermassive black holes, theoretical astrophysicist Lucio Mayer of the University of Zurich suggested December 15 at the Texas Symposium on Relativistic Astrophysics. The postulated process offers a major shortcut to supermassive status, as black holes are generally thought to start small and gradually grow by merging with each other and gobbling up matter. The mechanism also doesn’t rely on stars to spawn black holes in the first place.

Mayer’s proposal still has hurdles to clear before other astrophysicists accept it as viable. But if confirmed, it would solve the mystery of why astronomers keep spotting gargantuan black holes when the universe was less than a billion years old.

This supermassive conundrum boils down to timing. The first stars, some of them 100 times or more the mass of the sun, took shape a few hundred million years after the Big Bang. The largest ones exploded soon after and left behind black holes of roughly the same mass. Yet recent telescope observations reveal that by about 500 million years later, not very long on cosmic timescales, some black holes weighed in at 10 billion solar masses (SN: 4/4/15, p. 5). No matter how often ancient black holes feasted and combined forces, they would have had trouble growing by a factor of 100 million so quickly.

Mayer has tried to devise mechanisms that would birth jumbo black holes. The recipe requires getting huge amounts of matter to fall together until the collective gravity is strong enough to prevent light from escaping. Galactic gas seems like an ideal black hole–building ingredient, but it never seems to reach the necessary ultradense state; instead, it tends to cool and gather in small clumps that go on to become stars.

But if two primordial galaxies collided, Mayer proposes, then perhaps their gas wouldn’t have a chance to build stars. The galactic merger would spark turbulent swells that warm the gas and prevent it from clumping. A new set of computer simulations showed the growth of a dense disk of gas, immune to fragmenting into stars, that eventually became so compact that it collapsed into a black hole hundreds of millions of times as massive as the sun. Mayer calls this direct progression from gas to shadowy abyss “dark collapse.”

“There’s no light emitted,” he says. “It’s just a big black hole.”

Mitchell Begelman, an astrophysicist at the University of Colorado Boulder, says he likes Mayer’s line of thinking. But he worries that the factors stalling star formation, such as gas molecules rotating too quickly, would also prevent the disk from reaching critical mass. “I’m pretty skeptical you can get a collapse,” he says.

Mayer says he plans to run a more rigorous relativity-based simulation to see if anything halts the gravitational cave-in. Proving black holes were actually born big more than 13 billion years ago will be much harder, though the formation of such monsters should trigger potentially detectable ripples through space called gravitational waves.

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