Astronomers have discovered a new object that could help shed light on mysterious “little red dots” that were first spotted by the James Webb Space Telescope (JWST) in 2022.
The newfound object, dubbed “the Cliff,” suggests that the little red dots represent a totally new class of cosmic objects known as a “black hole star,” the researchers say. This newly hypothesized object would essentially be a black hole feeding so rapidly that it lights up the thick cocoon of gas surrounding it, making it glow like a star.
However, all of these theories are still evolving, so it’s unclear whether the dots are exotic objects or simply a stage in the growth of galaxies or black holes. When they were first discovered, little red dots were dubbed “universe breakers” because they seemed too old to exist in the first few billion years of the universe. Therefore, astronomers looked beyond the standard types of known objects to find an explanation for what they might be.
They proposed two models. “One possibility is that Little Red Dots are extremely massive and compact galaxies with intense star formation, leading to very large stellar densities in their cores,”said Fabio Pacucci, an astrophysicist at the Harvard & Smithsonian Center for Astrophysics who was not involved in the new study. This scenario suggests that little red dots are tiny-but-dense galaxies and rich in stars and that they involve exotic, never-before-seen processes.
“The other possibility is that they host massive black holes at their centers, often appearing ‘overmassive’ compared to the stellar mass of their galaxies,” he told Live Science in an email. In both cases, the redness would be due to the enormous dust surrounding the object.
The second explanation would mean that little red dots are galaxies that are powered by a massive black hole at their centers, like an active galactic nucleus (AGN). These black hole-fueled galaxies would be nothing like the other type of AGNs found in the early universe, known as quasars — extremely bright objects that are powered by large supermassive black holes and are easily detectable because they are not blocked by dust. The connection between these two types of populations remains unclear.
“Both explanations push the limits of our current understanding of early galaxy evolution,” Pacucci said.
A “Cliff”-hanger
In the new study, published Sept. 10 in the journal Astronomy & Astrophysics, a team of astronomers led by Anna de Graaff of the Max Planck Institute for Astronomy looked at a peculiar little red dot that existed 1.8 billion years after the Big Bang.
This little red dot, whose light took almost 12 billion years to reach us, was discovered among many other little red dots identified in the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES) obtained with JWST.
In the light of this object, the researchers noticed a very sharp jump in the brightness called the Balmer break. While this kind of rise is common in the light of different objects, the kind of sharpness seen in this object’s light could not be explained by massive galaxies or typical active galactic nuclei, researchers found. They identified it as an exaggerated version of a little red dot and dubbed it “the Cliff” for its sharp rise in the spectrum.
This unusually strong feature made astronomers wonder if they had seen something entirely new. The brightness of the object suggested a very energetic source, and the Balmer break originates from dense hydrogen gas at a specific temperature, de Graaff explained. These two hints led to the “black hole star” hypothesis.
“Black hole stars are [feeding] massive black holes that are surrounded by dense gas,” de Graaff explained. When black holes accrete surrounding matter, they emit a lot of light, and therefore heat the gas, making it glow and thus look like a star.
“The key difference, of course, is that normal stars are powered by nuclear fusion, which is not happening here,” de Graaff said. A black hole star can be thought of as a hot object wrapped inside an ultrathick blanket.
“The ‘black hole star’ hypothesis is certainly intriguing,” Pacucci said. “This work is interesting because it tries to bridge unexplained observational features of Little Red Dots with such theoretical ideas.”
Other little red dots may have similar signatures to the Cliff that may have gone undetected due to observational limitations, Pacucci said. However, the black hole star hypothesis is still at the beginning stage. Many more observations would be necessary to test the robustness of this scenario, and monitoring of these objects over time would help distinguish scenarios, Pacucci noted.
“We are not sure yet how they evolve into the black hole population that we see today,” de Graaff noted. “Because the number of little red dots decreases toward later cosmic times, it must be a short-lived phase.” Next, the team will use JWST to study brighter little red dots to understand the detailed structure of black hole stars.
If little red dots are, in fact, black hole stars, it could solve another puzzle. If black hole stars could grow at extremely rapid rates, it could explain the emergence of supermassive black holes very early in the universe.
The true nature of little red dots remains a mystery. If more cocooned black holes are discovered in the universe, researchers can find out if the little red dots are truly exotic black hole stars, a phase in a massive black hole’s growth, or simply a stage of galaxy evolution.
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