The origin of supermassive black holes has often been described as a "million-dollar question" for astronomy. If we base our models on how the universe is now they shouldn't have formed, and yet they are at the center of almost every galaxy.
A few solutions have been proposed, and a new formula might have the right ingredients to explain it all. In a paper, published in Nature Astronomy, an international team of researchers led by Dr John Regan from Dublin City University, Ireland, clarified what requirements the first galaxies needed to have to form supermassive black holes.
The framework they’re basing their hypothesis on is called “direct collapse black hole”. The idea is that primordial gas had to collapse directly into these gigantic objects as there’s not enough time for stars to form, explode, turn into black holes, and then merge into a supermassive black hole.
This is a very popular hypothesis and it has found some supporting evidence, but it has some issues. The collapsing gas needs to be stopped from turning into stars, and the researchers have shown that this can happen if the gas clouds are hot enough.
It might appear counterintuitive but stars can only form if the gas is cold enough to condense and fall onto itself due to gravity. But if the gas remains really hot it might collapse to form a large black hole without going through a stellar phase.
Rogan and his colleagues have worked out that if these gigantic gas clouds are surrounded by other primordial galaxies forming lots of stars, they will remain too hot to form stars. The intense UV radiation of these young bright stars should be enough to push the gas into a direct collapse black hole.
"The nearby galaxy can't be too close, or too far away, and like the Goldilocks principle, too hot or too cold," co-author Dr John Wise, an associate astrophysics professor at Georgia Tech in the US, said in a statement.
The protogalaxies they envision are actually quite small with the gas cloud forming the black hole being about 1,000 light-years away (about 1 percent of the Milky Way's size). According to their model, direct collapse black holes can form from within a few thousand to a few million years, which can explain how the furthest galaxies in the universe already have large supermassive black holes.
"Understanding how supermassive black holes form tells us how galaxies, including our own, form and evolve, and ultimately, tells us more about the universe in which we live," added Regan.
Supermassive black holes can be millions – if not billions – of times the mass of the Sun, and when feeding can be brighter than the combined light of an entire galaxy. Even the biggest ones are not much larger than our Solar System, but they play a crucial role in the evolution of their host galaxy.