Zeroing In on How Supermassive Black Holes Formed.

Computer-simulated image shows a supermassive black hole at the core of a galaxy. Credit: NASA, ESA, and D. Coe, J. Anderson, and R. van der Marel (STScI)

Supermassive black holes—objects containing hundreds of millions to billions of times the mass of a star—are one of the deepest mysteries of modern astrophysics. They lurk at the hearts of most large galaxies, including our own Milky Way. Given their ubiquity, these black holes may play a vital part in the formation and evolution of the universe. But how they grew so massive has long puzzled theorists worldwide.

The most sensible suggestion—that these monstrosities could only have grown so great by swallowing enormous quantities of gas over billions of years—is now known to be wrong. Recent observations have revealed the existence of black holes billions of times more massive than the sun just 800 million years after the big bang. And so, the riddle goes: How did they get there so quickly? Most astrophysicists agree supermassive black holes must stem from smaller “seed” black holes. They just don’t agree on how humble such a seed must be. One school of thought holds that the seed black holes should be big—thousands to several tens of thousands of times the sun’s mass; the other posits the seeds could be small—no heavier than a hundred solar masses.

Both camps must grapple with the fact black holes are messy eaters: Gravity can only cram so much gas down one’s maw before the material begins to pile up around it, forming white-hot disks that emit intense radiation and push additional incoming gas away, effectively cutting off the food supply. This is called the Eddington limit, and it is thought to severely hinder the rate at which any black hole can swallow matter and grow. The advantage of models using small seeds is that such welterweight black holes are relatively straightforward to make; the disadvantage is that to rapidly grow into supermassive black holes they must treat the Eddington “limit” as more of a suggestion, and rely on various potential exceptions to circumvent its constraints. Big-seed models, by contrast, respect the limit by giving growing supermassive black holes a huge head start to gobble up more gas before hitting it—but their bigger seeds are correspondingly more difficult to make. The giant clouds of gas that could collapse to form big seeds can also fragment into smaller clumps, forming clusters of stars rather than large black holes.

Whether advocating for big or little seeds, “there have been many theories that try to explain the existence or assembly of supermassive black holes, but none of them can give a natural solution,” says Naoki Yoshida, an astrophysicist at the University of Tokyo. Yoshida is a proponent of big seeds, and a co-author of a new study published Thursday in Science suggesting how they formed and gave rise to the early universe’s surprising population of supermassive black holes. His “natural solution” invokes high-speed streams of gas that flowed through the universe in the aftermath of the big bang as the critical catalyst. Specifically, it rests on the putative interaction between the zooming gas and dark matter—the mysterious invisible substance that seems to act as gravitational glue for galaxies.

GROWING A BLACK HOLE

Along with his collaborators at The University of Texas at Austin and the University of Tübingen in Germany, Yoshida used computer simulations to re-create conditions in the early universe by feeding the program cosmological parameters such as the density of dark matter, which astronomers have calculated from measurements of the early universe’s makeup. “We tried to reproduce this initial state as close to the real observations [as we could],” Yoshida says, “and we let that universe evolve over time.”
According to the group’s simulations, in some parts of the universe the gravity from dark matter would have ensnared fast-moving streams of primordial hydrogen and helium left behind by the big bang. In its aftermath, researchers recently discovered, these early gases accelerated in some areas to incredible speeds—or “really fast winds,” as Yoshida calls them. “You can just imagine it’s very hard to trap a gas that is moving very fast,” Yoshida says. Picture placing your hand against the jet from a fire hose, he says—your arm would just swing back with the force. “The only way to stop this strong wind is really to cause strong enough gravity,” he says. And the researchers calculate that in every three-billion-light-year expanse of the early universe, there was a large enough clump of dark matter with enough gravity to suck in and trap this wind—like having sufficient strength to push the jet of water back in the opposite direction. This attraction between the gas and the dark matter created a large gas cloud and prevented small stars from forming along the way.
This simulated gas cloud then collapsed into a massive star, which continued to swallow more gas until it reached 34,000 times the sun’s mass. This unusually hypothetical massive star could only reach such magnitudes if it was made purely of hydrogen and helium—the two elemental gases that swirled around the early universe before any stars underwent supernova explosions that created heavier elements such as carbon, nitrogen and oxygen. The idea for a massive star had been proposed before, but this is the first time a group simulated it. “Our computer simulation really showed this kind of phenomena actually happens and that this kind of monstrous star can actually be formed,” Yoshida says. After reaching this gigantic mass the star finally collapsed and the seed for a supermassive black hole was born. “We didn’t really look for a nice solution; [it] came as a natural result,” he says. “That’s why I think this is really the final solution, at least to the question of the origin of [supermassive black holes].” Not everyone agrees, however.

GOOD ANSWER, BUT…

Other scientists who favor the big-seed hypothesis have different ideas about how those seeds form in the first place. A recent study published in Nature Astronomy, for example, proposes such seeds form not through the murky motions of dark matter but rather via the behavior of ordinary stars in galaxies. In this scenario intense bursts of ultraviolet light from vigorous star formation in a nearby young galaxy could prevent stars from forming in a giant gas cloud, allowing it to remain whole long enough to directly collapse into a black hole with a mass of up to 100,000 suns.

John Wise, an astrophysicist at Georgia Institute of Technology and co-author of the Nature Astronomy study, thinks this new work is an important step forward in the field because Yoshida and his colleagues were the first to simulate the effects of these early gas motions on the formation of supermassive black holes. But he says it does not rule out his own theory. “I think there are multiple pathways [for] how to form these supermassive black holes,” he says. “This [pathway] is just another, and I think it’s totally possible.” Yet he does note that it is rare to find such fast-moving gases in the early universe. “These velocities do fluctuate [depending on] where you are in the universe, so there is still some low probability of this actually happening.” The chances of coming across an area of the universe with such a fast-moving wind are only 0.3 percent, according to Yoshida. Conversely, he and his colleagues note, giant gas clouds directly adjacent to young star-manufacturing–galaxies also seem like rare occurrences. “The net probability of this event happening is really uncertain,” Yoshida says.
Greg Bryan, an astrophysicist at Columbia University and senior author of the Nature Astronomy paper praises the new findings. “This is not a definitive answer, but it’s the best by far looking at this particular mode for black hole formation,” he says. He is a bit worried, however, about how close their simulations were to forming a smaller set of stars. For a black hole to form, a bunch of these early gases needs to gather in a very small region, which wouldn’t happen if they dispersed to form a bunch of stars. If the conditions in the simulation changed a tiny bit, he says, it wouldn’t form the massive seed. “On the other hand, as much as I trust any of the models, I trust theirs,” Bryan adds.
Fulvio Melia, an astrophysicist at the University of Arizona, is not as ecstatic about this theory. “There are a lot of unknown physics that [the authors] have to rely on, just like all the other proposals [for] forming massive seeds or having these objects grow at a very high rate,” he says. “They have to make specific assumptions about how the dark matter behaves, but we don’t [even] know what it is.”

ENDING THE SEEDS OF DOUBT

To definitively answer this question of how these massive beasts came to be, these scientists all point to the future possibility of observing the “seeds” in the early universe using advanced next-generation telescopes. This possibility may not be that far off. There are several initiatives such as the proposed ATHENA mission from the European Space Agency set to launch in 2028, which aims to detect x-ray emissions from these supermassive giants. NASA’s upcoming James Webb Space Telescope, slated to fly next year, could also provide insights with its studies of the universe’s first stars and galaxies.
“The exciting thing is that there is a way of testing these ideas over the next few years because people will go out and do a thorough search over the whole sky for these objects,” Melia says. As to why there’s so much discussion, he adds: “What people are proposing is something different than what we know about in [parts of] the universe closer to where we are.”