Hungry black holes eat two courses at once

How do some black holes grow to be billions of times the mass of the sun? Perhaps they funnel in food by tilting their plates.

Nearly every galaxy holds a supermassive black hole at its heart. "We know they managed to grow very quickly after the big bang," says Andrew King at the University of Leicester, UK. Supermassive black holes were already around when the universe was only a billion years old, less than a tenth of its current age.

A black hole could grow just by sucking in gas. In simple models, gas forms a disc around the black hole and spirals in, but far too slowly: bringing food to the hole's gaping maw from within just a few light years would take longer than the age of the universe. "We need a faster mechanism," says Chris Nixon, also at Leicester.

Crash and fall

The researchers wondered what happens if gas comes in from different directions. Nixon, King and their colleague Daniel Price simulated two discs orbiting a single supermassive black hole at different angles. Where the two discs meet, their gas particles collide and rob angular momentum from each other.

"If you have two guys on motorbikes going around the Wall of Death and they collide, they'll lose centrifugal support and they'll fall," King says. The same thing happens to the gas, and it plunges directly in towards the black hole.

The simulations showed that black holes can grow up to 1000 times faster if they are fed from two tilted discs.

Whether that really explains the growth of supermassive black holes depends on the actual flow of gas inside galaxies in the early universe, King says. "It is still an unsolved problem, but I think it's quite promising that if you allow the accretion to be chaotic, you make things a lot easier."

Journal reference: Monthly Notices of the Royal Astronomical Society, in press

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