This is a problem that has led astronomers on their heads for a long time. Our current knowledge suggests that in this time frame, only so-called intermediate mass black holes up to 100,000 times the mass of our Sun should have been able to grow. And while various theories have been proposed for this rapid early growth of black holes, the answer remains elusive.
“This is still a big problem in astrophysics,” says John Regan, an astrophysicist at Dublin City University, Ireland.
Black holes form after a massive star runs out of fuel, sometimes as a result of a supernova and other times without a supernova, which is called a direct collapse scenario. Once a star has no fuel left to burn, it can no longer support its mass and collapses. If the mass of the star is large enough, it will collapse into an object with an immense gravitational pull from which nothing, not even light, can escape: a black hole.
As the black hole gradually draws in more and more nearby dust and gas, it can grow in size, eventually reaching the giant proportions of a supermassive black hole, like the first one photographed in April 2019. Scientists are now investigating whether the Supermassive black holes could have formed from supermassive stars that collapsed to form large “seed” black holes, giving them a growth advantage.
Regan coordinated a project called SmartStars , which used one of Ireland’s most powerful supercomputers, ICHEC, to model how supergiant stars could provide the seeds for supermassive black holes. The team wanted to see if these stars could explain the rapid growth of supermassive black holes, which we see today at the center of almost all galaxies.
250.000
They found that such stars could grow up to 250,000 times the mass of the sun in 200 million years after the Big Bang, a tempting result. However, even supercomputers have their limitations. The researchers were only able to model the future of such stars for a million years, but modeling must cover 800 million years to see if these stars could actually be the seeds of supermassive black holes.
“It’s a really great starting point,” Regan clarifies. “During the next generation of supercomputers we will be able to take those simulations further and further.”
Other theories for how these black holes grew so rapidly are that a small fraction of the black holes grew at an incredible rate, or that the smaller black holes merged to become a supermassive black hole.
Muhammad Latif, an astrophysicist at the University of the United Arab Emirates in Abu Dhabi, agrees with Regan that the supermassive star model remains our best theory at the moment. Latif was principal investigator of the FIRSTBHs project which, like SmartStars, investigated the plausibility of the supermassive star model, using simulations on a supercomputer in France.
His project, which was carried out at the CNRS in France, showed that supermassive stars could produce seed black holes hundreds of thousands of times the mass of our sun. “We found this method to be basically feasible,” Latif said, explaining that these initial seed black holes are large enough to explain the growth of billion-solar-mass supermassive black holes in a short period of time.
However, conditions in the early universe are required to be right for these black holes to form . Large amounts of material made of hydrogen and helium would be needed to form enough massive seed black holes to produce supermassive black holes, which appears to have been possible.
But other inexplicable factors mean this is still an open question. Seed black holes would need to attract matter at a rate of at least 0.1 solar masses per year, for example, and at the moment it is not clear if this is possible.
Observatories
Several observatories are already allowing us to probe black holes in the early universe in great detail. In October 2019, astronomers announced that they had used the Atacama Large Millimeter / submillimeter Array (ALMA) in Chile to find a thick ring of dust and gas around a supermassive black hole within a distant galaxy. With two streams of gas spinning in opposite directions, it is believed that this ring could have fed the supermassive black hole with enough material to make it grow rapidly.
Previously, in August 2019, NASA’s Chandra X-ray Observatory managed to detect a so-called “covert” black hole that was growing rapidly when the universe was only 6% of its current age. A thick cloud of gas hides the black hole and its resulting quasar, a glowing region of superheated material that surrounds it, but Chandra was able to detect it by seeing X-rays emerging from the cloud.
However, future telescopes will likely be needed to study the rapid growth of supermassive black holes in even more detail. For example, although we can predict the existence of seed black holes, we still cannot see them. NASA’s upcoming James Webb Space Telescope (JWST), due to launch in 2021, may be able to detect some of the undiscovered seed black holes.
Meanwhile, the European Space Agency’s ( ATHENA) Advanced High Energy Astrophysics Telescope, due to launch in 2031 , should give us a better understanding of how supermassive black holes arise.
“Everyone is very hopeful that we will get a much better image with the ATHENA mission,” says Latif. And maybe soon, we will finally know how these huge objects grew so much in such a short time.
“It’s like going to kindergarten and finding a eight-foot-tall baby.”
Original article
This article was originally published in Horizon, the EU Research and Innovation Magazine
