Supermassive Black Holes: How Did They Form In The Very Early Universe?

Black holes that form due to the collapse of massive stars typically have masses 5-20 times that of the sun, but supermassive black holes — found in the centers of nearly all known sizeable galaxies — are far bigger, at about hundreds of thousands, or even billions, of solar masses. Given the 13.8 billion years that have passed since the Big Bang, it may be enough time for supermassive black holes to grow to their gigantic sizes, but how then do we explain that some of them formed less than 800 million years after the universe came into existence?

Astronomers have struggled to answer that question since these oldest supermassive black holes, in the last decade or so, were determined to be over 13 billion years old. An associate professor from University of Helsinki, Peter Johansson, proposes a new theory to solve this enigma in a paper published Monday in the journal "Nature Astronomy".

“The observations of extremely massive black holes in the very early Universe are somewhat surprising, since it is not straightforward to grow the mass of black hole from tens up to billions of solar masses in the limited time available,” Johansson said in a statement Tuesday.

Read: Two Supermassive Black Holes Found Hiding In Our “Cosmic Backyard”

In the paper, titled “Rapid formation of massive black holes in close proximity to embryonic protogalaxies,” he argues that the ancient supermassive black holes could not have formed through the conventional method of accretion of gas. Accreting such a large amount of gas in such a short time would have led to too much radiation, which would have pushed back the infalling gas.

Johansson uses a newer, alternative explanation of the formation of supermassive black holes to model his own theory on. This newer explanation, the so-called “direct collapse black hole model,” suggests that very large gas clouds — of between 10,000 and 100,000 solar masses — collapsed directly to become the seeds of the black holes. However, if the cooling of the gas leading to the collapse were efficient, the collapsing gas cloud would fragment and form stars.

Therefore, a prerequisite for the formation of supermassive black holes under this theory is very inefficient cooling, which in the very early universe could only be achieved by emission of molecular hydrogen. And Johansson shows in the paper that “the near simultaneous formation of two galaxies can lead to a situation in which the radiation from the first galaxy can destroy the molecular hydrogen in the second galaxy just at the right time.”

“In this way, a massive direct collapse black hole seed can form in the second galaxy, which can evolve rather quickly to a billion solar mass black hole by the time they are observed in the universe,” he said in the statement.

John Regan of Dublin City University, as well as other researchers from Ireland and the United Kingdom collaborated in the research.