Astronomers know that supermassive black holes at the centres of galaxies existed in the early universe, but how these objects managed to accumulate such heft in a short cosmological timespan is a mystery. Now, a team of researchers in Germany and the US has used a humongous computer simulation to show that cold streams of gas from outside a young galaxy could have fed its central black hole fast enough for the hole to grow rapidly.

Supermassive black holes are furnaces at the centres of galaxies. They suck in vast amounts of matter – which releases energy that causes the gas that surrounds them to glow. Astronomers call these glowing galactic centres quasars, and the UK Infrared Telescope Deep Sky Survey (UKIDSS) has found light from a quasar that was emitted as little as 800 million years after the Big Bang. This quasar and several picked up by the Sloan Digital Sky Survey are considerably brighter than expected. Indeed, they emit so much light that the black holes at their centres must have been enormous, at least a billion times the mass of the Sun.

Assuming that a supermassive black hole begins life as a relatively small black hole at the collapsed core of a massive supernova, Volker Springel of the Heidelberg Institute of Theoretical Studies in Germany says that it would need to have fed at its maximum rate from birth onwards in order to reach a billion solar masses now. "It seems possible, but it's a bit contrived," he says. This is because the rate at which a black hole accumulates matter is proportional to its mass, and therefore small black holes grow very slowly.

Direct collapse

An alternative explanation is that a very large amount of gas – roughly 100,000 solar masses – may have collapsed directly into the black hole. Now, Springel and colleagues – including team leader Tiziana Di Matteo at Carnegie Mellon University in the US – have used a computer simulation to show that this scenario is possible.

The team modelled the universe in a virtual box 2.4 billion light-years to a side – a volume that is roughly 1% of the visible universe today. This size of simulation was chosen in order to increase the chances that extremely massive quasars would emerge from the model. Inside the box, gas and dark matter, a form of matter that interacts through gravity alone, were represented by 65.5 billion particles.

"It's a remarkable achievement to be able to simulate such a huge volume of space to the precision needed to say something about a single black hole," says Daniel Mortlock of Imperial College London. While the resolution of the study was good enough to look at individual black holes, it had to be coarse enough to make the simulation feasible. As a result, each gas "particle" had the mass of 57 million Suns, while dark matter weighed in at 280 million solar masses per particle.

Billion-year simulation

The simulation covered the timespan from 10 million years after the Big Bang to about 1.3 billion years later. As time progressed, gravity caused the particles to gradually clump together. Once a congregation of gas particles reached a density associated with black-hole formation, the program introduced a particle of 100,000 solar masses into the middle of the clump to represent a black hole. This "seed" could then begin accreting gas particles according to a model of black-hole growth.

After 800 million years, one black hole had reached 3 billion solar masses, while nine more were close to the billion-solar-mass mark. To find out how they had grown, the team zoomed in on them, finding that those growing the fastest appeared to be fed by dense streams of gas. This picture supports the idea of "cold gas flows" penetrating directly to the black hole without warming up through interactions with the hot gas already in the vicinity. Although black-hole mergers have been proposed as a route to supermassive black holes, merged black holes were not among the largest in the simulation.

"[The simulation] is the first to quantitatively estimate that cold gas flows can deposit large quantities of fresh 'fuel' to the centre of galaxies, possibly feeding supermassive black holes even in absence of mergers," says Lucio Mayer of the University of Zürich, Switzerland. "However, the resolution of the simulations is still too low to ascertain if such gas would directly feed the central black hole." He suspects that it would be more likely to settle into the disc of gas surrounding the black hole, feeding it more slowly, but this detailed behaviour must be explored with higher-resolution simulations.

The research is reported in The Astrophysical Journal.