Just 700 million years after the Big Bang, when the universe was still in its infancy, we are already seeing supermassive black holes weighing 1 billion suns. How could they grow so fast? A team of astronomers is using computer simulations to see what the formation of these dark monsters might have looked like.
How to make a supermassive black hole
If you want to create a billion-solar-mass black hole from scratch, to borrow a phrase, you have to start with a star — or maybe just the gas that stars are made of.
While the first stars in the universe could have created the first black holes, these would have been relatively small on a supermassive scale, with masses of only around 100 suns. Perhaps the first stars clustered together, and when stars formed black holes, those black holes merged and then merged again. Even then, such black hole “seeds” would have been only 1,000, maybe 10,000 solar masses. These black holes should have grown very quickly to become supermassive in such a short time.
But there is another way: Some astronomers have put forward the idea that in the small, early universe, when gas was dense and pristine, gas clouds could collapse directly into more massive black holes.
The calculations for such massive implosions are tricky, however. What keeps the pieces of the gas cloud from cooling and collapsing under their own weight, as star-forming clouds usually do in the modern universe?
Some astronomers have suggested that ultraviolet emission from nearby infant stars may have heated the gas, keeping it too hot to fragment. Others have argued that such specific requirements would make the process too rare to explain the number of supermassive black holes we have already found in the young universe.
To stay together
Today, Muhammed Latif (UAE University), Daniel Whalen (University of Portsmouth, UK and University of Vienna) and their colleagues report in Nature that massive black holes can form without these special conditions.
The discovery is based on computer simulations, which reconstruct the conditions of the infant universe, when it was less than 100 million years old. Simulations are necessary because this era of the first stars is beyond the reach of our current telescopes.
The simulation tracked the growth of a small sea of foaming material fed by four torrents of incoming gas. While such nodes would have been common in the web of material filling the universe, Whalen says such flows were unusual because they carried so much gas. Latif adds that the rivers of gas were not only dense but fast; rushing at speeds of 50 km/s (over 100,000 mph), they carried between 1 and 10 suns of material per year.
The sea in the center of these streams of matter developed, and in the sea a tuft took shape, then another. The turbulence of the incoming gas streams prevented the massive clusters from immediately collapsing into stars; instead, the clumps continued to grow. At the end of the simulation – 1.4 million years later – they contained tens of thousands of solar masses.
Eventually, these clumps compress into what researchers call supermassive stars; tracking their evolution requires another type of computer simulation, which takes into account stellar physics. Stellar monstrosities don’t last long in this simulation, just 1 million years, before collapsing again into black holes of 30,000 and 40,000 solar masses, respectively.
Such massive seeds could easily collect more gas and become the dark behemoths seen by astronomers. Even though the type of confluence explored in this study is rare, Latif, Whalen and colleagues believe it would occur often enough to explain the observations.
“The novel cold flux-filled environment that is being explored numerically in this study is very exciting,” says Priya Natarajan (Yale), “because it appears to provide a natural pathway for the formation of massive black hole seeds.”
But that’s not the only scenario that leads to a direct meltdown, she warns. Natarajan, who was not involved in the current study, explored a different scenario in 2014, finding that a dense star cluster could similarly allow direct collapse. “The result is that there are multiple pathways to rapidly magnify and create massive seeds of black holes in situ and early in the universe.”
Upcoming observations from the James Webb Space Telescope, she adds, will help distinguish between different black hole seed scenarios. Webb won’t be able to detect supermassive stars, even if they’re millions of times brighter than the Sun, but it’s possible he could detect the seeds of black holes that continue to grow when the universe is less than 200 million years.