Copenhagen researchers make the front page of Nature: Solving the mystery of the universe's ‘little red dots’

Since December 2021, when the James Webb super telescope saw first light, some 1.5 million kilometres from Earth, researchers around the world have been scratching their heads over unexplained red dots among stars and galaxies in the images taken by the telescope.

The so-called ‘little red dots’ can be seen when the universe was “only” several hundred million years old, and a billion years later, they seem to disappear again. So what were they?

Some scientists argued that they were massive galaxies, powerful enough for the James Webb Space Telescope to detect them 13 billion years later. But that theory did not fit well with how long these galaxies took to evolve after the Big Bang – they came later.

However, after two years of continuous analysis of images with the red dots, researchers from the Niels Bohr Institute's Cosmic Dawn Centre have found the explanation in the most powerful phenomenon in our universe: Black holes. The red dots have thus given researchers insight into how the universe's first black holes were born.

"The little red dots are young black holes, a hundred times less massive than previously believed, enshrouded in a cocoon of gas, which they are consuming in order to grow larger. This process generates enormous heat, which shines through the cocoon. This radiation through the cocoon is what gives little red dots their unique red colour," says Professor Darach Watson, one of the principal researchers behind the study and adds:

‘“They are far less massive than people previously believed, so we do not need to invoke completely new types of events to explain them.”

The discovery has landed the researchers from the Cosmic Dawn Centre on the front page of one of the world's largest scientific journals, Nature.

Black holes are 'messy eaters'

There are now hundreds of known little red dots, which we now know to be young black holes. Although they are some of the smallest black holes ever discovered, they still weigh up to 10 million times more than the Sun and have diameters of ten million km.

Black holes swallow everything in their vicinity and grow as they eat. But because the event horizon of black holes is not very large, the infalling gas heats up to such high temperatures that it shines brightly and delivers more energy than any other process we know of. This intense radiation causes much of the matter that the hole is consuming to be blown back out .

"When gas falls towards a black hole, it spirals down into a kind of disk or funnel towards surface of the black hole. It ends up going so fast and is squeezed so densely that it generates temperatures of millions of degrees and lights up brightly. But only a very small amount of the gas is swallowed by the black hole. Most of it is blown back out from the poles as the black hole rotates. That's why we call black holes ‘messy eaters’," explains Darach Watson.

Explanation of rapidly growing black holes

At the centre of our own galaxy, the Milky Way, there is a supermassive black hole four million times the mass of the Sun, and the same is true of all other galaxies. But black holes and their role in the universe are still shrouded in mystery.

The new discovery sheds new light on the early development of black holes and provides some of the answer to how, just 700 million years after the Big Bang, there could already be supermassive black holes with masses up to a billion times greater than the Sun.

“We have captured the young black holes in the middle of their growth spurt at a stage that we have not observed before. The dense cocoon of gas around them provides the fuel they need to grow very quickly,” says Darach Watson.

The study is authored by V. Rusakov, Darach. Watson, G. P. Nikopoulos, Gabriel Brammer, R. Gottumukkala, T. Harvey, Kasper Elm Heintz, R. Damgaard, S. A. Sim, Albert Sneppen, A. P. Vijayan, N. Adams, D. Austin, C. J. Conselice, C.M. Goolsby, Sune Toft, J. Witstok.

Link to paper