In recent years, astronomers have discovered something extraordinary, as we were able to look deeper and deeper into the early Universe.
Before the universe was a billion years old, giant black holes until more than a billion times the mass of the sun was already formed somehow† Given what we know about the formation and growth of black holes, the presence and size of these behemoths is extremely challenging to explain. How did they get there so soon after the big bang† And how did they get so big?
Now, supercomputer simulations have revealed an origin that explains how they formed without the need for exotic conditions: rare reservoirs of turbulent cold gas that collapsed into stars more massive than anything in the universe today. These would have been the huge seeds that grew into supermassive black holes.
“We find supermassive black holes at the centers of most massive galaxies today, which can be millions or billions of times the mass of the sun. But in 2003 we started finding quasars — highly luminous, actively accreting supermassive black holes that resemble cosmic lighthouses in the early universe – which existed less than a billion years after the big bang”, said cosmologist Daniel Whalen from the University of Portsmouth in the UK.
“No one understood how they formed at such early times. This discovery is particularly exciting because it has destroyed 20 years of thinking about the origin of the first supermassive black holes in the universe.”
There are two main theories about how supermassive black holes form. The first is the bottom-up model. A single massive star dies, usually leaving behind a black hole with a mass about 100 times the mass of the sun.
Over time – a lot of time – the black hole guzzles up a lot of material and gets bigger and bigger until it is millions to billions of times the mass of the sun. This is extremely difficult to reconcile with quasars in the early universe.
The other option is if you start with a really huge black hole ‘seed’, up to 100,000 times the mass of the sun. The stars that collapsed to form these black holes would have had a very, very short cosmic life, perhaps 250,000 years, before collapsing into a black hole.
There are no known stars of that mass around todayand we know of no current formation mechanism that could produce them. But simulations have shown that in the early Universe, when conditions were quite different from today’s conditions, such stars could theoretically have formed at the intersections of rare but powerful flows of dense, turbulent, cold gas.
Cosmologists thought it required some really exotic conditions, such as backgrounds of strong ultraviolet radiation, or supersonic flows between gas and dark matter† And none of these exotic conditions resembled the environments in which these early universe quasars were found.
Led by astrophysicist Muhammad Latif of the University of the United Arab Emirates in the UAE, the researchers ran simulations of the gas flows and were pleased to find that supermassive black holes formed spontaneously at the intersections of these flows, without the need for exotic circumstances .
In the simulation, the turbulence generated by the intersecting flows prevents the formation of normal stars like the ones we see today. Usually this happens when a dense tangle of material in a cold cloud collapses under gravity to form a baby star, but when there is too much turbulence, the conditions are not stable enough for this to happen.
Ultimately, however, the cloud in the simulation became so massive that it catastrophically collapsed into two giant stars, clocking in at 31,000 and 40,000 times the mass of the sun.
As gas from the streams continues to feed the clouds, in just a few hundred million years, a supermassive black hole can form and grow billions of times the mass of the sun.
“Therefore, the only primordial clouds that could form a quasar just after cosmic dawn – when the first stars in the universe were formed – also easily created their own massive seeds. This simple, beautiful result explains not only the origin of the first quasars, but also their demographics – their numbers in the early times,” Walloon concluded:†
“The first supermassive black holes were simply a natural result of structure formation in cold dark matter cosmologies — children of the cosmic web.”
The research was published in Nature†
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