A mysterious class of small, red objects in the early universe might be explained by black holes inside dense cocoons of gas, like a star.
Since launching in 2021, the James Webb Space Telescope (JWST) has found hundreds of objects in the early universe that appear extremely red and compact, dubbed little red dots (LRDs).
About a billion years after the big bang, LRDs seem to vanish. This means they might be related to a process fairly early in the cosmos, and could be the progenitors of some galaxies – but their properties aren’t what astronomers expected.
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“We thought we knew what the earliest galaxies would be like,” says at the American Museum of Natural History. “We did not think they would be like this. So we’re all just coming up with the weirdest possible ideas.”
at the Massachusetts Institute of Technology and his colleagues propose one such idea. They suggest some LRDs might be supermassive black holes that are surrounded by extremely dense gas comparable to the outer regions of a star’s atmosphere. The black holes would be eating this material voraciously, causing the gas to glow brightly.
Naidu and his colleagues identified an LRD in existence 660 million years after the big bang that they think fits this scenario. They declined to speak to żěè¶ĚĘÓƵ about the work while their paper is under review.
According to the team’s analysis, the ball of gas around the black hole would be slightly wider than Pluto’s orbit around our sun, with a mass millions of times that of our star. The black hole itself would have a hot, superheated disc of material around it, producing huge amounts of light and energy that burst through the cocoon of gas. From our point of view, that would appear as a little red dot.
“I personally really like the solution,” says from the University of Texas at Austin. “It’s very elegant. It doesn’t [require] any exotic physics, and it doesn’t break the universe.”
Another team led by at the University of Manchester, UK, who also didn’t want to comment for this story, has put forward a similar explanation for LRDs in separate work. “They are all proposing this kind of embedded black hole in a very dense atmosphere,” says at the University of Cambridge.
There are other possible solutions. Bellovary suggests LRDs could be explained by black holes at the same time. “Little red dots have a lot of similar characteristics to a black hole eating a bunch of stars,” she says.
Alternatively, LRDs might be explained not by black holes, but by incredibly dense collections of stars. However, you would have to pack a huge amount of stars – equivalent to 10 billion suns – into a very small area to fit the observed properties. “You need to squeeze all this mass within 100 light years,” says Ji, making a cluster so dense “you expect the stars to collide with each other”. Only a few thousand stars are found within a similar distance from our sun.
If LRDs are explained by black holes, they could reveal how supermassive black holes – the extremely dense objects found at the centre of galaxies like our own – grew in the early universe. “It could finally answer how you make a supermassive black hole,” says Bellovary. “The earlier we look [in the universe], there’s still supermassive black holes. At some point they have to form. How the heck do you do that?”
Upcoming observations could help pin down an explanation. Several programmes to investigate LRDs in more detail have been one of which is led by Taylor. He will be looking for spectral lines representing absorption of light by calcium that might point towards or against the black hole solution.
If stars are the culprit, “I’d expect to see these calcium lines”, he says. “However, if we see this cocoon of gas argument, it’s much less likely we would see these lines.”
arXiv
arXiv
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