Black hole stars really do exist in the early universe

The early Universe appears to be littered with huge star-shaped balls of gas, fueled by a black hole at their core. The discovery took astronomers by surprise and may solve one of the the biggest mysteries caused by discoveries from the James Webb Space Telescope (JWST).

When JWST first began looking back at the first billion years of the universe, astronomers found a group of what appeared to be extremely compact, red and very bright galaxies, unlike any we can see in our local universe. The most popular explanations for these so-called little red dots (LRDs) suggested that they were either supermassive black holes with dust swirling around them, or galaxies very densely filled with stars – but neither explanation fully accounted for the light that JWST detected.

Earlier this year, astronomers instead proposed that LRDs are dense spheres of gas with a black hole at the center. called black hole stars. “When material falls into a black hole, a lot of gravitational energy is released, and this can cause the entire ball of gas around it to glow like a star,” says Anna de Graaf at Harvard University. While the energy doesn't come from nuclear fusion like in a normal star, the end effect is a similar glowing ball of dense gas, only on a much larger scale, billions of times brighter than our Sun, de Graaf says.

However, although there were some promising LRDs that supported this interpretation, it was still controversial.

Now de Graaf and her colleagues have analyzed the widest sample of LRDs since JWST began its observations, including more than a hundred galaxies, and concluded that they are best explained by star-like objects or black hole stars. “The name 'black hole star' is of course still controversial, but I think there is now a decent consensus in the community that we are looking at an accreting black hole, shrouded in dense gas,” says de Graaf.

When the team examined the brightness of light at different frequencies, called a spectrum, coming from the LRD, the samples best matched light coming from a single relatively smooth surface, called a blackbody. This is how stars appear, as opposed to the more complex and sharp spectra seen in galaxies, which emit light from a combination of stars, dust, gas and a central black hole.

“The black hole star model has been around for a while, but it was thought to be so weird and unusual, but it actually seems to work and makes the most sense,” says Gillian Bellovary at the American Museum of Natural History in New York.

“When you use a black hole star model, it really simplifies things,” says Anthony Taylor at the University of Texas at Austin. “It's just a simple structure, but it explains [observations] very, very beautiful, without any real exotic physics.”

In September, de Graaf and her colleagues also found a separate single LRD it had an extremely sharp peak in the frequency of light coming from the galaxies, which they nicknamed “The Cliff”. “We saw certain features in the spectrum that really couldn't be explained by any of our existing models,” says de Graaf. “When you have that, for the first time you can say with confidence that we need to move away from both of these pictures that we've been looking at. We need to look at something else.”

While many astronomers now agree that LRDs appear to function like huge stars, it will be difficult to prove that their source is a black hole, de Graaf says. “The center of this object is encased in a very, very dense or optically thick shell. What's inside is hidden by what's around it,” says de Graaf. “We think they're black holes just because these objects are so luminous.”

One way to prove they are black holes is to look at how the light coming from them changes over time, and see if they change in the same way that we know black holes in our local universe do, says Sihan Ji at Cambridge University. “You see brightness changing over relatively short periods of time, like months or even days, but for these little red dots, there seems to be very little evidence of that variability most of the time.”

It may be difficult to look for evidence of longer lasting changes in light from the LRD since JWST has limited time to make observations, but another recent study Could give some guidance. Fengwu San at Harvard University and his colleagues discovered an LRD whose light was bending around a very massive galaxy located between it and Earth, and called it a gravitational lens. The lens produced four images of the original LRD, but because the light from each image traveled different distances to reach us, each was equivalent to looking at the galaxy in different images over a 130-year period.

The four images appear to show brightness variability similar to known pulsating stars, but hinting at a much larger width, again consistent with the black hole star hypothesis. Sun and his team refused to talk to New scientist for this story.

While the idea of ​​using a gravitational lens to measure the LRD at different times is reasonable, there may be other explanations for the brightness changes, Bellovary says. “I'm not sure there's enough data to really back up their claim. I'm not saying their claim is wrong, but I think the variations could be explained by some other things.”

If these galaxies do turn out to be black hole stars, they will require entirely new models of how they came into being and what these black holes will eventually become, says de Graaf, because we don't see any equivalent systems in our local universe.

“Essentially, this could be some kind of new growth mode or part of the growth history of these supermassive black holes,” she says. “Whether they go through just one of these events, or how long their life span is, or how significant their contribution is. [to the final mass of the black hole] It’s still very unclear.”

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