Formation of massive BHs

Supersonic gas streams enhance the formation of massive black holes in the early universe

Science  29 Sep 2017

Thermal evolution of the collapsing cloud. In each run, the cloud became unstable at the points marked by the circles.

The first supermassive Black Holes

Supermassive black holes existed less than a billion years after the Big Bang. Because black holes can grow at a rate that depends on their current mass, it has been difficult to understand how such massive black holes could have formed so quickly. Hirano et al. developed simulations to show that streaming motions—velocity offsets between the gas and dark matter components—could have produced black holes with tens of thousands of solar masses in the early universe. That’s big enough to grow into today’s supermassive black holes.

Science, this issue p. 1375

Abstract

The origin of super-massive black holes in the early universe remains poorly understood. Gravitational collapse of a massive primordial gas cloud is a promising initial process, but theoretical studies have difficulty growing the black hole fast enough. We report numerical simulations of early black hole formation starting from realistic cosmological conditions. Supersonic gas motions left over from the Big Bang prevent early gas cloud formation until rapid gas condensation is triggered in a protogalactic halo. A protostar is formed in the dense, turbulent gas cloud, and it grows by sporadic mass accretion until it acquires 34,000 solar masses. The massive star ends its life with a catastrophic collapse to leave a black hole—a promising seed for the formation of a monstrous black hole.

Recent discoveries of super-massive black holes (SMBHs) at redshift z ~ 7, when the universe was just 5% of its present age, pose a serious challenge to the theory of black hole (BH) formation and evolution. The physical mechanisms that form the BH seeds and drive their growth are not yet known but must explain how an initial seed BH can reach a mass of 10 billion times that of the Sun within 1 billion years after the Big Bang. It is thought that the mass growth is a self-regulating process, limited by the so-called Eddington rate that is proportional to the BH mass; therefore, starting from a massive BH holds a key to the rapid formation of SMBHs.

Sloshing, supersonic gas may have built the baby universe’s biggest black holes

By Joshua Sokol |

Joshua Sokol is a journalist based in Boston.

A central mystery surrounds the supermassive black holes that haunt the cores of galaxies: How did they get so big so fast? Now, a new, computer simulation–based study suggests that these giants were formed and fed by massive clouds of gas sloshing around in the aftermath of the big bang.

“This really is a new pathway,” says Volker Bromm, an astrophysicist at the University of Texas in Austin who was not part of the research team. “But it’s not … the one and only pathway.”

Astronomers know that, when the universe was just a billion years old, some supermassive black holes were already a billion times heavier than the sun. That’s much too big for them to have been built up through the slow mergers of small black holes formed in the conventional way, from collapsed stars a few dozen times the mass of the sun. Instead, the prevailing idea is that these behemoths had a head start. They could have condensed directly out of seed clouds of hydrogen gas weighing tens of thousands of solar masses, and grown from there by gravitationally swallowing up more gas. But the list of plausible ways for these “direct-collapse” scenarios to happen is short, and each option requires a perfect storm of circumstances.

For theorists tinkering with computer models, the trouble lies in getting a massive amount of gas to pile up long enough to collapse all at once, into a vortex that feeds a nascent black hole like water down a sink drain. If any parts of the gas cloud cool down or clump up early, they will fragment and coalesce into stars instead. Once formed, radiation from the stars would blow away the rest of the gas cloud.

One option, pioneered by Bromm and others, is to bathe a gas cloud in ultraviolet light, perhaps from stars in a next-door galaxy, and keep it warm enough to resist clumping. But having a galaxy close enough to provide that service would be quite the coincidence.

The new study proposes a different origin. Both the early universe and the current one are composed of familiar matter like hydrogen, plus unseen clumps of dark matter. Today, these two components move in sync. But very early on, normal matter may have sloshed back and forth at supersonic speeds across a skeleton provided by colder, more sluggish dark matter. In the study, published today in Science, simulations show that where these surges were strong, and crossed the path of heavy clumps of dark matter, the gas resisted premature collapse into stars and instead flowed into the seed of a supermassive black hole. These scenarios would be rare, but would still roughly match the number of supermassive black holes seen today, says Shingo Hirano, an astrophysicist at the University of Texas and lead author of the study.

Priya Natarajan, an astrophysicist at Yale University, says the new simulation represents important computational progress. But because it would have taken place at a very distant, early moment in the history of the universe, it will be difficult to verify. “I think the mechanism itself in detail is not going to be testable,” she says. “We will never see the gas actually sloshing and falling in.”

But Bromm is more optimistic, especially if such direct-collapse black hole seeds also formed slightly later in the history of the universe. He, Natarajan, and other astronomers have been looking for these kinds baby black holes, hoping to confirm that they do, indeed, exist and then trying to work out their origins from the downstream consequences.

In 2016, they found several candidates, which seem to have formed through direct collapse and are now accreting matter from clouds of gas. And earlier this year, astronomers showed that the early, distant universe is missing the glow of x-ray light that would be expected from a multitude of small black holes—another sign favoring the sudden birth of big seeds that go on to be supermassive black holes. Bromm is hopeful that upcoming observations will provide more definite evidence, along with opportunities to evaluate the different origin theories. “We have these predictions, we have the signatures, and then we see what we find,” he says. “So the game is on.”