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Ancient galaxies colloquially known as "little red dots" have proven a mystery ever since astronomers discovered them three years ago. Now, a new study finds the strange features of little red dots might be explained by supermassive black holes in disguise during their youth.

With the help of NASA's $10 billion James Webb Space Telescope (JWST), astronomers first discovered the mysterious specks of light known as little red dots at the end of 2022. They only existed for a short time in the cosmos, first appearing in the universe less than 1 billion years after the Big Bang and almost completely disappearing after 2 billion years, explained study lead author Vadim Rusakov at the University of Manchester in England. (The universe is currently about 13.8 billion years old.)

The discovery ignited a fierce debate among scientists over the identity of the little red dots. One possible explanation for these ancient bright spots was that they were extraordinarily star-rich galaxies. Another possibility was that little red dots hosted supermassive black holes — light in the galaxies may have emerged from gas that became super-hot as it rushed toward the enormous gravitational pull of these black holes.

A key problem with these possible explanations, however, was that both proposed objects were both too massive to have formed so early in the history of the universe. In addition, supermassive black holes should emit X-rays and radio waves, and scientists have detected neither from little red dots.

In the new study, researchers investigated 12 ancient galaxies to get a better sense of the nature of little red dots. The earliest of these galaxies existed when the universe was only about 840 million years old.

Their analysis suggested that little red dots "are simply too luminous and too compact to be explained by a large number of stars," Rusakov told Space.com. "If they were purely made up of stars, they would be the densest galaxies in the universe."

Instead, the research team's model suggested the most luminous sources of light they examined were as bright as more than 250 billion suns but also less than a third of a light-year across. This is much smaller than a galaxy — the distance from our sun to its nearest neighbor, Proxima Centauri, is about 4.25 light-years. The compact sizes of these incredibly bright spots within little red dots suggest they must be supermassive black holes.

The spectrum of radiation emitted from the little red dots suggested that before the JWST detected these rays of light, they got scattered off electrons in dense clouds of ionized gas in the centers of the little red dots. Such cocoons would trap most of the radiation generated near black holes.

"These objects turned out to be supermassive black holes despite missing almost all typical indications of massive black holes," Rusakov said. "They have an almost perfect disguise that removes X-ray and radio emission."

By analyzing the light from the little red dots, the scientists calculated the speed of the light-emitting gas within most of the dots as being about 670,000 miles per hour (1.08 million kilometers per hour). Assuming this gas was orbiting the black holes at the centers of these little red dots, they could deduce the black holes were likely about 100,000 to 10 million times the mass of the sun. This is about 100 times less than previous estimates suggested, and is closer to what researchers would expect from young super-massive black holes early in the history of the cosmos.

"Our results imply, most importantly, that for the first time we are seeing supermassive black holes early in their lifetimes, possibly early enough to understand how they were born—either by continuously growing from smaller black holes or by starting big, as intermediate-mass black holes that formed from collapsing streams of gas," Rusakov said.

Future research may shed light on how these supermassive black holes were born. "If we are lucky, little red dots may still preserve clues from the time when they were formed — whether it’s the gas chemistry or some useful physical property of the black holes and their cocoons that can help to differentiate between different theories," Rusakov said. "This is one of the biggest remaining questions in astrophysics and it seems that we are closer than ever to being able to answer it."

The scientists detailed their findings in the Jan. 15 issue of the journal Nature.