Astronomers Aaron Smith and Volker Bromm of The University of Texas at Austin, working with Avi Loeb of the Harvard-Smithsonian Center for Astrophysics, have discovered evidence of an unusual kind of black hole born extremely early in the universe. They showed that a recently discovered unusual source of intense radiation is likely powered by a “direct-collapse black hole,” a type of object predicted by theorists more than a decade ago.
An image based on a supercomputer simulation of the cosmological environment where primordial gas undergoes the direct collapse to a black hole. The gas flows along filaments of dark matter that form a cosmic web connecting structures in the early universe. The first galaxies formed at the intersection of these dark matter filaments.
These direct-collapse black holes may be the solution to a long-standing puzzle in astronomy: How did supermassive black holes form in the early epochs of the universe? There is strong evidence for their existence, as they are needed to power the highly luminous quasars detected in the young universe. However, there are several problems that should prevent their formation, and the conventional growth process is much too slow.
Astronomers think they know how supermassive black holes weighing in at millions of suns grow in the heart of most galaxies in our present epoch. They get started from a “seed” black hole, created when an extremely massive star collapses. This seed black hole has the mass of about 100 suns. It pulls in gas from its surroundings, becoming much more massive, and eventually may merge with other seed black holes. This entire process is called accretion. In 2003, Bromm and Loeb came up with a theoretical idea to get an early galaxy to form a supermassive seed black hole, by suppressing the otherwise prohibitive energy input from star formation. Astronomers later dubbed this process “direct collapse.”
Smith, Bromm, and Loeb had become interested in a galaxy called CR7, identified from a Hubble Space Telescope survey. Various unusual features in the spectrum, such as the absence of any detected lines from elements heavier than helium, together with the source’s distance, meant that it could either be a cluster of primordial stars or a supermassive black hole likely formed by direct collapse. Smith ran simulations for both scenarios using the Stampede supercomputer at UT Austin’s Texas Advanced Computing Center. The type of modelling Smith used is called “radiation hydrodynamics,” according to Bromm. In the simulations, the star cluster scenario “spectacularly failed,” while the direct collapse black hole model performed well.
The team's work is about more than understanding the inner workings of one early galaxy. “With CR7, we had one intriguing observation," Bromm said. We "are trying to explain it, and to predict what future observations will find. We are trying to provide a comprehensive theoretical framework.”
(Source: Royal Astronomical Society)