James Webb telescope spots 'stingray' galaxy system that could solve the mystery of 'little red dots'

Astronomers have spotted an intriguing triple-galaxy system, nicknamed "The Stingray," that dates to when the universe was just over 1.1 billion years old. A new analysis of the celestial sea creature has revealed an object that may provide clues about the nature of mysterious cosmic objects dubbed "little red dots" (LRDs).

LRDs were first observed in 2022 by the James Webb Space Telescope (JWST). Astronomers initially proposed that these compact red objects, which seem to permeate the very early universe, could be galaxies that host actively feeding black holes known as active galactic nuclei (AGNs). Alternative LRD theories involve ancient supermassive stars on the verge of collapse and exotic black hole stars.

In the new study, published March 9 in the journal Astronomy & Astrophysics, astronomers reconstructed the recent star formation history of the triple-galaxy stingray. They found that interactions between galaxies may have pushed an AGN into an unusual state resembling a transition into or out of a little red dot. Astronomers dubbed the galaxy that hosts this unusual AGN a ‘transitional little red dot’ (tLRD).

"We have all the necessary ingredients to produce such a transition: starbursts caused by galaxy interactions, an AGN, and a galaxy (tLRD) whose spectral features match almost all LRD criteria," lead study author Rosa María Mérida, an astrophysicist who studies galaxy formation and evolution at Saint Mary's University in Canada, told Live Science in an email.

The unique system's nickname came from its appearance: In early images, it resembled a stingray with a body, head and tail. However, later analysis revealed that the "tail" was formed by unrelated distant objects and had aligned by chance in the image.

The Stingray is made of three galaxies: a Balmer break galaxy that is relatively massive and evolving more steadily, a tLRD, and a satellite star-forming galaxy that is less massive and appears to have joined the system more recently.

Back to the past

Due to observational limitations, the researchers could not definitively determine how the three-galaxy system formed. Instead, they proposed a scenario based on indirect evidence. They did this by reconstructing the galaxies' star formation histories, using data from the Canadian NIRISS Unbiased Cluster Survey, one of the deepest JWST surveys to date.

By comparing these histories across galaxies and incorporating relative stellar masses, the team looked for patterns that might indicate past interactions. For example, if multiple galaxies showed changes in star formation at similar timescales, that could point to a shared event, such as a close encounter. Additionally, lower-mass galaxies with weaker gravity are more susceptible to disturbance, which may trigger bursts of star formation.

The team's analysis suggested that about 100 million years ago, the tLRD galaxy experienced a burst of star formation, which was likely triggered by an interaction with the nearby Balmer break galaxy. The more massive Balmer break galaxy, however, appeared largely unaffected and evolved steadily. Later, around 10 million years ago, the smaller satellite galaxy experienced increased star formation.

"We think this is the moment when the [satellite] galaxy entered the Stingray system," Mérida noted.

Around that time, some activity was observed in tLRD but not in the Balmer break galaxy. By this stage, tLRD also would have been quite massive, making this behavior difficult to explain through gravitational interactions alone. This raises the question of what drove the activity in tLRD, while the Balmer break galaxy shows little change in its star formation history. This suggests factors beyond simple gravitational interactions may be at work.

Part AGN, part LRD

The researchers proposed that the answer may lie in the behavior of the central black hole. Mérida explained that interactions between galaxies can trigger bursts of star formation, but the activation of an AGN can occur later. In this scenario, the earlier encounter may have first sparked star formation and then, with some delay, fueled the black hole in tLRD, pushing the galaxy into its unusual state.

The active black hole in tLRD shows spectral features of a type I AGN characterized by a bright and unobscured core. But it is also compact and bright in ultraviolet light, partly resembling a little red dot. However, it lacks one key spectral signature that almost all observed little red dots have in their light spectrum: a V-shaped feature. So it looks like a mix of both objects but not completely like either.

"This galaxy is strategically in between the little red dot population and compact Type I AGN," Mérida said. Therefore, tLRD is part AGN and part LRD, but it's unclear whether it is entering or exiting the LRD phase.

"The paper supports the idea that at least some little red dots are evolutionary phases rather than a wholly distinct class," Devesh Nandal, a postdoctoral researcher at the Harvard and Smithsonian Center for Astrophysics who was not involved in the study, told Live Science in an email. "The system is physically compact, spectroscopically confirmed, and the authors infer enhanced recent growth in the tLRD and [satellite galaxy]," compared what would be expected from their normal, internal processes, making their interaction-driven interpretation credible. However, while galaxy interactions may trigger or shut down the LRD phase, they do not fully explain the black hole's mass or the LRD phenomenon as a whole, Nandal noted.

What next?

If this transition phase is very short — less than about 5 million years — the chances of spotting a galaxy in that stage are very low, Mérida said. In that case, tLRD might just be a normal AGN. But if the transition lasts longer, astronomers should find many such transitional objects in current galaxy surveys. That means researchers need to do two things: carefully search existing data for more candidates, and improve theoretical models to predict how often these transitions happen and determine how to clearly identify them.

A larger sample size of such "in-between" objects and a better understanding of how long the AGN spends in active and quiet phases can establish the new results more robustly, Nandal said. A clear distinction between how the black hole is currently feeding and how the black hole originally formed is also crucial, he said. For example, the black hole may have already existed as a massive seed from a supermassive star or other origin; in that case, the LRD-like activity we observe now likely reflects later fueling or dust obscuration rather than the black hole forming from scratch.

The team plans to conduct follow-up studies on The Stingray and other LRDs found in the Canadian NIRISS Unbiased Cluster Survey. If confirmed, this transitional object would support the idea that little red dots are not a separate class of objects but a temporary phase in the evolution of a black hole system, with their behavior controlled by their surroundings.

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