James Webb Space Telescope discovers most distant ring galaxy ever, hinting at ultra-rare early universe collision

When a small galaxy plunges through the center of a larger galaxy, the impact creates a wave of dense gas that ripples outward, similar to how water moves when a stone is dropped into a pond. As a result, a collisional galaxy is formed. Gas piles up along the wave and forms stars, producing a ring. These galaxies are rare in the local universe, and the few candidates found in the distant universe have been hard to confirm.

Several clues support the case for GS18660 as a collisional ring galaxy. For one, the researchers spotted a small companion galaxy, moving away from GS18660 at about 264 miles per second (425 kilometers per second). They also detected a bridge of gas linking the two, and the velocity along it changed, Perna noted. The change in velocity along this bridge-like structure indicates that the companion and GS18660 may be interacting.

Its position is also important, Perna added ‪—‬ the companion is located roughly along the main galaxy’s minor axis, consistent with where an intruder would be expected to appear after a head-on plunge through the galaxy’s disk.

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Using the companion’s speed and distance, the team estimated a rough timeline for the impact. It revealed that the collision happened about 17 million to 42 million years earlier in the galaxy’s history. This time scale seems reasonable, Conselice said.

He explained that it is difficult to determine the age of a past interaction directly, but that if one did occur, it must have happened around the time a rapid star-formation episode began and its age should be consistent with the average ages of the star clusters.

Looking at star formation rates with different instruments, the team estimates that 10% to 20% of the stellar mass in the ring formed in the past few tens of millions of years. This suggests that the collision and the ring’s burst of new stars might have happened at the same time, so they were most likely caused by the same event, Perna explained.

“It is not, by itself, definitive proof of a collision, but together with the bridge and the ring morphology, it strengthens the case for a recent impact,” Perna noted.

The discovery may also shed light on how common these kinds of collisions used to be. Whereas earlier estimates suggested that collisional rings in the early universe might be about as rare as they are today ‪—‬ around 1 in 10,000 galaxies ‪—‬ the new findings hint that head-on collisions that form ring galaxies may be more common in the early universe.

“If GS18660 is confirmed as a collisional ring, it would therefore suggest that we need to revisit our estimates of how frequently these systems occur in the distant universe,” Perna said

The team expressed caution about how much the data can prove. “It is really the combination of several pieces of evidence rather than one single observation,” Perna said. “So we regard the collisional interpretation as our preferred scenario rather than a definitive conclusion.”

Conselice thinks the evidence presented in the paper is consistent with a genuine collisional origin for the ring. “It looks to me like the merger hypothesis is more likely, given the nearby galaxy intruder and the stability of the intruder galaxy and the main galaxy itself,” Conselice said. “Overall, it is convincing evidence that a past interaction and/or merger has occurred for this galaxy system based on the light distribution.”

The other competing explanation involves a stellar bar. It proposes that the ring was not made by an intruder at all. GS18660 also shows a bar-like structure running north to south, and bars can funnel gas and stars into rings at specific distances from the center. Perna said the team cannot yet tell whether this is a true stellar bar or a result of the interaction.