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After using the James Webb Space Telescope (JWST) to peer back toward the beginning of time, astronomers have proposed a radical new explanation for one of the universe's most flummoxing phenomena.
Little red dots (LRDs) are mysteriously compact, brilliant celestial objects found predominantly when the universe was less than 10% of its current age.
As revealed by JWST's unmatched infrared sensitivity, LRDs emerged incredibly early, only around 600 million years after the Big Bang, and then began disappearing about a billion years later.
Now, in a paper published in The Astrophysical Journal Letters, astronomers have proposed a novel formation mechanism for LRDs: They may be birthed by previously undiscovered celestial companions, whose intense ultraviolet (UV) radiation causes gas clouds to collapse into incredibly dense and exotic objects, like "black hole stars."
"The most surprising aspect… is that these little red dots are not just 'red dots,' but there is a more complex emission nearby and around them," Josephine Baggen, an astronomer at Yale University and first author of the study, told Live Science in an email. "We think those what we call 'companions' are starlight."
Connecting the dots
In the study, the researchers compiled a sample of 83 LRDs, imaged with JWST, from ultradeep surveys. They found that 36 of the 83 LRDs, including over 80% of the brightest ones, hosted at least one companion that shined bright in blue-ish ultraviolet light; little red dots with little blue companions.
These companions had masses ranging from hundreds of millions to billions that of the sun, suggesting that they may be star clusters or relatively small early galaxies, Baggen told Live Science.
These UV-spewing companions spurred the formation of LRDs from immense gas clouds, the team's new proposal states. Normally, cold molecular gas clouds fragment and condense into stars. But the intense UV irradiation from the companions halted the fragmentation process, potentially squeezing the gas clouds into supermassive stars that directly collapsed into black holes while skipping the explosive supernova stage that normally marks these stars' deaths.
This proposal helps to explain why LRDs appear bright in red optical light and UV wavelengths, with a dip between the two caused by the wavelengths of light absorbed by hydrogen gas. Rather than originating from a single object, the optical red light derives from the LRD, the UV light comes from its nearby companion, and the dip is thought to be caused by the dense cocoon of gas around the LRD.
As a result, the researchers suggested that all LRDs may have such partners but they may be too close together to be distinguished. Conversely, some companions may be separated by greater distances than the researchers accounted for in this study, requiring future observations to zoom out.
Galactic potential?
Intriguingly, this work may illuminate a couple of early-universe enigmas. First, the black holes manifesting from these interactions may be between 100,000 and 1 million solar masses, forming the "seeds" necessary to explain how ancient supermassive black holes grew so surprisingly massive so early in cosmic history.
Additionally, because LRDs may merge with the relatively small, UV-spewing galaxies that birthed them, this process may have created the impressively immense galaxies all around us today.
"We do think that this might be the birth of the supermassive black holes around these UV companions, born 'outside the galaxy,' [which] will eventually merge," Baggen told Live Science. "Whether this is what happened to the Milky Way in its earliest phases, we cannot really say, but it is plausible! There is still a lot of debate about how LRDs evolve and what they turn into at later times."
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