Mystery of supermassive black holes solved
Quasars are supermassive black holes with enormous luminosity. How they formed in the early days of the universe has always puzzled astronomers. Now they have found an explanation. Cold gases play an important role here.
BAlmost a billion years after the Big Bang, the first quasars ignited in the cosmos – supermassive black holes in the centers of newly formed galaxies. But how could such large objects with up to a billion solar masses arise in such a short time – from an astronomical point of view?
An international team of researchers has now found an answer to this question with the help of computer simulations: streams of cold, turbulent gas condense to form the first black holes with ten to one hundred thousand solar masses. These objects then serve as “seeds” for the formation of supermassive black holes, the scientists write in the journal “Nature”.
In today’s cosmos, almost every galaxy has a large black hole at its center with millions to billions of times the mass of our sun. At first, astronomers thought these supermassive black holes would increase in mass more or less evenly throughout cosmic history. However, this notion had to be corrected when numerous quasars were detected in the early cosmos. They, too, are supermassive black holes into which matter flows and heats up in the process – which is why quasars shine brighter than the galaxies in whose centers they are located.
“Cosmological simulations have shown that these quasars can be formed by the influx of cold gas,” explain Muhammad Latif from the University of the United Arab Emirates and his colleagues from Austria, Great Britain and Canada. “But the prerequisite for this is that black holes with ten to one hundred thousand solar masses already exist. So far, however, there has been no conclusive explanation for their formation.”
Although unusual scenarios can lead to the formation of these “seeds” for supermassive black holes – but far too rarely to explain the frequency of quasars in the young cosmos.
Latif and his colleagues have now succeeded in using high-resolution computer simulations to find an explanation for the formation of the first black holes. As the scientists report, the influx of cool gas into the galaxies forming in the young cosmos leads to strong turbulence – and this turbulence prevents stars from forming from the gas in the normal way.
Only when the mass of the collected cool gas has increased to 30,000 to 40,000 solar masses does the dense gas cloud collapse under its own gravity and form a correspondingly large black hole.
According to Latif and his colleagues, this simple, robust process ensures that wherever there is sufficient gas for the formation of a quasar, a “seed” can initially form. And this process also occurs frequently enough to explain the number of quasars. “The first quasars were therefore a natural consequence of the formation of structures in the early cosmos,” say the researchers, “and did not require exotic, finely tuned environments, as previously thought.”