At the center of almost every galaxy is a supermassive black hole weighing millions, if not billions, of times the mass of the Sun. The origin of these enormous objects is still very much a mystery, but a group of Italian astrophysicists might have found an important clue in solving it.
Fabio Pacucci from the Scuola Normale Superiore in Pisa, Italy, and his colleagues have detected two very interesting objects, both of which are a smaller type of black hole that they believe may be the seeds of supermassive black holes. These objects were detected thanks to NASA’s Chandra X-ray Observatory, Hubble Space Telescope, and Spitzer Space Telescope, and they have an estimated mass of more than 10,000 times the Sun.
The study has been accepted for publication in the Monthly Notices of the Royal Astronomical Society, with a pre-print available online at arXiv. These objects are so-called "direct collapse black holes," which form directly from giant gas clouds collapsing and not from single stars going supernova.
It is possible to create supermassive black holes from smaller stellar ones by having many black holes merge together and accrete gas from their environment, but this process requires a long time. The universe already had large supermassive black holes when it was less than 1 billion years old, when it would have been impossible to build up a black hole of 1 billion solar masses via this method.
“It’s a billion dollar problem," Pacucci told IFLScience. "We observe black holes of a billion solar masses less than 1 billion years after the Big Bang."
The problem has a theoretical solution. About a decade ago, scientists suggested that it is possible for a supermassive black hole to form very quickly if it was made from intermediate-mass black holes, with a typical mass around 100,000 times the Sun.
The conditions to form these intermediate-mass objects were only present in the early universe. Large gas clouds today tend to fragment and form small stars. The fragmentation is only possible if the cloud is cool, and the “coolants” are elements heavier than helium – i.e. metals – and molecular hydrogen. In the early universe, there were no metals, so the first stars were very big and very bright. The light of these stars destroyed molecular hydrogen, so the clouds couldn't cool down, instead collapsing into a very large black hole.
“Our research suggests we have observed the first two examples of direct collapse black holes in the early universe,” added Pacucci. “Direct collapse black holes are among the first objects to form in the universe. They are not small like stellar black holes, they are not huge like supermassive black holes. They are the missing link between these two classes.”
The team plans to conduct follow-up observations of these objects to confirm that they show the expected properties of supermassive black hole seeds. Future observatories like the James Webb Space Telescope and the European Extremely Large Telescope will hopefully look even further into the universe, and discover many more of these objects.