Feeding black holes

9 June 2016

Supermassive black hole
Credit: - NRAO/AUI/NSF; Dana Berry/SkyWorks; ALMA (ESO/NAOJ/NRAO)

Experts show supermassive black hole preparing for a ‘chaotic’ feast one billion light years from Earth

Astronomers from Cardiff University are part of an international team that has, for the first time, detected billowy clouds of cold, clumpy gas streaming towards a supermassive black hole at speeds of up to 800,000 miles per hour and feeding into its bottomless well.

The observation, which was aided by the work of Dr Timothy Davis from the School of Physics and Astronomy, marks the first direct evidence to support the theory that black holes feed on clouds of cold gas.

Using one of the most powerful telescopes in the world – the Atacama Large Millimeter/submillimeter Array, or ALMA – the team found that the feeding process was “chaotic” and “clumpy”, as opposed to a smooth, simple and clean process that was previously hypothesized.

The latest results have been published today, 9 June, in the journal Nature.

Dr Davis, who played a key role in preparing and analyzing the exquisite ALMA data that made this discovery possible, said: “It was magical being able to see evidence of these clouds accreting onto the supermassive black hole. At that very moment, nature gave us a clear view of this complicated process, allowing us to understand supermassive black holes in a way that has never been possible before.

“The data has provided us with a snapshot of what is happening around the black hole at one precise time, so it’s possible that the black hole has an ever bigger appetite and is devouring even more of these cold clouds of gas surrounding it.”

Previous models have suggested that the gradual growth of supermassive black holes – a process known as accretion – occurs when surrounding hot gas accumulates smoothly onto the black hole, much like a slow graze.

But the very first observations made by Dr Davis and the team suggest that in addition to this, supermassive black holes may occasionally quickly gobble up faster-moving cold gas as it comes nearby.

Professor Michael Macdonald, a co-author of the paper from MIT’s Kavli Institute for Astrophysics and Space Research, said: “This diffuse, hot gas is available to the black hole at a low level all the time, and you can have a steady trickle of it going in. Every now and then, you can have a rainstorm with all these droplets of cold gas, and for a short amount of time, the black hole’s eating very quickly. So the idea that there are these two dinner modes for black holes, is a pretty nice result.”

The research team used ALMA to observe a distant galaxy one billion light years away. The galaxy, called Abell 2597, spans some tens of thousands of light years across and is one of the brightest in the universe.

The team were interested in discerning how many stars were being born in the galaxy and therefore went about measuring cold gas – stars are formed when cold gas collapses.

To the team’s surprise, they ended up discovering something quite unexpected at the centre of the galaxy around a supermassive black hole – the shadows of three very cold, clumpy gas clouds.

The three gas clouds were cast against bright jets of material spewing from the black hole, suggesting that these clouds were very close to being consumed by the black hole.

“We got very lucky,” Professor MacDonald continued. “We could probably look at 100 galaxies like this and not see what we saw just by chance. Seeing three shadows at once is like discovering not just one exoplanet, but three in the first try. Nature was very kind in this case.”