James Webb Telescope discovers ‘Stingray’ galaxy that could solve ‘little red dot’ mystery

Astronomers have discovered an intriguing triple galaxy called the Stingray that dates back to when the universe was just over 1.1 billion years old. A new analysis of creatures in the celestial sea has revealed an object that may provide clues about the nature of a mysterious cosmic object called a “little red dot” (LRD).

The LRD was first observed in 2022 by the James Webb Space Telescope (JWST). Astronomers originally proposed that these compact red objects that appear to pervade the very early universe could be galaxies hosting actively feeding black holes known as active galactic nuclei (AGNs). Alternative LRD theories include ancient supermassive stars on the verge of collapse and exotic black hole stars.

In a new study published March 9 in the journal Astronomy & Astrophysics, astronomers reconstructed the recent star-forming history of the triple galaxy stingray. They discovered that interactions between galaxies may have caused the AGN to enter an unusual state where it moves in and out of small red dots. Astronomers named the galaxy containing this unusual AGN the Transitional Little Red Dot (tLRD).

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“We have all the ingredients needed to trigger such a transition: starbursts caused by galaxy interactions, AGNs, and galaxies whose spectral signatures match nearly all LRD criteria (tLRDs),” study lead author Rosa Maria Merida, an astrophysicist who studies galaxy formation and evolution at St. Mary’s University in Canada, told Live Science via email.

This unique system’s nickname comes 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 an unrelated, distant object that coincidentally lined up in the image.

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

go back to the past

Observation limitations have prevented researchers from definitively determining how the three galaxies formed. Instead, they proposed a scenario based on indirect evidence. They did this by reconstructing the galaxy’s star formation history using data from Canada’s NIRISS Unpolarized Galaxy Cluster Survey, one of the deepest JWST surveys to date.

By comparing these histories across galaxies and incorporating relative stellar masses, the researchers looked for patterns that might indicate past interactions. For example, if multiple galaxies show changes in star formation on similar timescales, it could indicate a common event, such as a close encounter. Additionally, low-mass galaxies with weaker gravity are more susceptible to disturbances, which can trigger bursts of star formation.

The researchers’ analysis shows that about 100 million years ago, the tLRD galaxy experienced an explosion of star formation, likely caused by an interaction with the nearby Balmer Blake galaxy. However, the larger Balmer-break galaxies were largely unaffected and appeared to be evolving steadily. Then, about 10 million years ago, star formation increased in smaller satellite galaxies.

“We think now is the moment. [satellite] “The galaxy has entered the Stingray system,” Merida pointed out.

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Around that time, some activity was observed in tLRD, but not in the Balmer Blake galaxy. By this stage, the tLRD would have also become very large, making it difficult to explain this behavior in terms of gravitational interactions alone. This raises the question of what caused the tLRD activity, but the Balmer Break galaxy shows little change in its star formation history. This suggests that factors beyond simple gravitational interactions may be at play.

Part AGN, part LRD

The researchers proposed that the answer may lie in the behavior of the central black hole. Merida explained that interactions between galaxies can trigger bursts of star formation, but activation of the AGN may occur later. In this scenario, a previous encounter could have first triggered star formation and then, a short time later, accelerated the tLRD’s black hole, pushing the galaxy into an anomalous state.

Active black holes in tLRD exhibit the spectral signature of a type I AGN, characterized by a bright and indistinct core. However, it is also compact and bright under UV light, partially resembling a small red dot. However, one important spectral feature is missing that nearly all observed small red dots have in the spectrum of light. It is a V-shaped feature. Therefore, both objects appear to be mixed, but not exactly similar to one or the other.

“This galaxy is strategically located between a cluster of small red dots and a compact type I AGN,” Merida said. Therefore, tLRD is part of AGN and part of LRD, but it is unclear whether it is entering or exiting the LRD phase.

“This paper supports the idea that at least some small red dots are an evolutionary step rather than an entirely separate class,” Devesh Nandal, a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics who was not involved in the study, told Live Science via email. “The system is physically compact and spectroscopically confirmed, and the authors say that tLRD and [satellite galaxy]” and compared it with what would be expected from normal internal processes in galaxies, increasing the credibility of the interaction interpretation. However, Nandal pointed out that while galactic interactions can trigger or stop the LRD phase, they do not completely explain the black hole’s mass or the LRD phenomenon as a whole.

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If this transitional phase is very short (less than about 5 million years), Merida said, the chances of discovering a galaxy at that stage are very low. In that case, tLRD might just be a regular AGN. But if this transition lasted longer, astronomers could find many such transition objects in current surveys of galaxies. This means researchers need to do two things. One is to carefully search existing data to find more candidates, and the other is to refine theoretical models to predict how often these transitions occur and to determine how to unambiguously identify them.

With larger sample sizes of such “intermediate” objects and a better understanding of how much time AGNs spend in active and resting phases, new results can be established with more certainty, Nandal said. He said it is also important to clearly distinguish between how black holes currently feed and how they were originally formed. For example, black holes may have already existed as giant seeds from supermassive stars or other origins. In that case, the LRD-like activity we currently observe would likely reflect later fueling or dust shielding, rather than the black hole forming in the first place.

The research team plans to conduct follow-up studies on stingrays and other LRDs found in Canada’s NIRISS unbiased cluster study. Identification of this transient object would support the idea that the little red dot is not a distinct type of object, but a temporary step in the evolution of a black hole system whose behavior is controlled by its surroundings.

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