At the center of almost all galaxies, there are gigantic black holes, millions if not billions of times the mass of our Sun. The formation of these objects happened very early in the history of the universe, but the exact mechanism is not clear. However, scientists believe they have now found a big clue.
Astronomers from the University of Texas and Harvard have discovered that a "direct-collapse black hole" is the most likely explanation for an incredibly bright light source seen in the early universe. These findings are published in the Monthly Notices of the Royal Astronomical Society.
There are a few theoretical ways to form very massive black holes. One is that the hydrogen gas from the Big Bang fell into the wells of gravity created by dark matter. There it created the first stars in the universe, which were hundreds if not thousands of times the mass of the Sun.
The supermassive black holes might have come from these first stars, or maybe by the gas itself, collapsing directly on itself due to the intense gravity – hence, a direct collapse black hole. Collapsing gas clouds tend to radiate away energy, which allows them to fragment into stars. But if the gas remains hot and the mass of the cloud is large enough then the star formation doesn’t happen.
These conditions were unique to these early epochs, and so our understanding is quite limited.
"The ultraviolet photons keep the gas hot, thus suppressing any star formation," said study co-author Volker Bromm in a statement. "These are the desired, near-miraculous conditions: collapse without fragmentation! As the gas gets more and more compact, eventually you have the conditions for a massive black hole."
The team was interested in studying CR7, one of the brightest objects from the early universe, and an ideal candidate for early stars and early black holes. In this galaxy, they discovered unusual features only explained by sources hotter than 100,000°C (180,000°F).
The features could be explained either by a direct-collapse black hole or by a cluster of early massive stars. The researchers ran simulations to establish the most likely scenario, and the idea of a star cluster "failed spectacularly." This is in agreement with the recent NASA’s Chandra observations of potential direct collapse black holes.
"With CR7, we had one intriguing observation. We are trying to explain it, and to predict what future observations will find. We are trying to provide a comprehensive theoretical framework," said Bromm.
The recent discovery of more galaxies like CR7 and the launch, in the next few years, of more powerful telescopes will allow for more observations, but at this point, the direct collapse scenario seems to be the right one.