Fireballs in the Perseus Cluster: Euclid Spots Star Formation in Stripped Galaxy Tails

In a recent study led by Koshy George, astronomers used early data from the European Space Agency's Euclid space telescope to examine how galaxies evolve in dense environments. They focused on ram-pressure stripping (RPS), a process in which a galaxy moving through the hot gas of a cluster loses its own gas. Without gas, a galaxy typically stops forming stars. However, this study found evidence that RPS may actually spark new star formation—but not inside the galaxy. Instead, stars are forming in long, faint tails of stripped gas trailing behind galaxies.

Targeting Galaxies in the Perseus Cluster

The researchers looked at two galaxies in the Perseus Cluster, a large grouping of galaxies located around 240 million light-years away. The galaxies, named UGC 2665 and MCG +07-07-070, show signs of unusual tail-like features extending from their discs. These features aren’t from gravity pulling stars apart (as in galaxy mergers) but instead resemble gas tails created by pressure, similar to wind blowing against a moving object. These comet-shaped structures contain bright knots, or clumps, which Euclid’s sharp resolution revealed to be small star-forming regions only about 100 parsecs across.

Ruling Out Gravity as the Culprit

To figure out what caused these features, the team compared the effects of gravity and ram pressure on the galaxies. Using models of the Perseus Cluster’s mass and gas distribution, they calculated that the pressure from the cluster’s hot gas was much stronger than the gravitational pull holding gas in the galaxies. This meant the gas could be stripped away. Meanwhile, they found no strong nearby galaxies close enough to have caused the features through gravitational interaction. Euclid’s deep imaging also showed no diffuse stellar tails or shells, which would have been expected if gravity were responsible.

Multi-Wavelength Confirmation

The team didn’t rely on Euclid alone. They also used ultraviolet (UV) observations from UVIT, optical images from the Canada-France-Hawaii Telescope (CFHT), and radio data from the LOFAR telescope. The UV and Hα images showed active star formation in the stripped tails. The LOFAR data revealed radio tails trailing behind the galaxies—evidence of supernova explosions from newly formed stars. These tails were well-aligned with the optical features, reinforcing the idea that the gas was stripped by pressure, then formed stars in place.

Comet Tails in Motion

Interestingly, the direction of the tails gives clues about the galaxies' orbits. UGC 2665 has a tail pointing away from the cluster center, suggesting it's currently falling inward. In contrast, MCG +07-07-070 has a tail pointing toward the cluster center, hinting that it may have already passed through and is now swinging back. This kind of motion fits the theory that the galaxies are on their first or second pass through the cluster, picking up speed and encountering denser regions of gas.

Star Formation in Hostile Environments

One surprising result is the presence of star formation in such harsh conditions. The gas in galaxy clusters is extremely hot—millions of degrees—making it difficult for cold gas clouds to survive, let alone collapse into stars. But in these galaxies, the stripped gas somehow manages to cool and clump together, creating small, isolated star-forming regions far from the galactic disc. These features, sometimes called "fireballs", have been seen before, but Euclid’s sharp vision allows them to be studied in unprecedented detail.

A Glimpse into Galaxy Evolution

This study demonstrates the power of Euclid's high-resolution, low-light imaging to observe how galaxies change over time in dense environments. By combining Euclid with other telescopes, astronomers are gaining new insights into how external forces like ram pressure can reshape galaxies—not just by halting star formation, but by triggering it in unexpected places. As Euclid continues to map the sky, it will likely uncover many more examples, helping scientists understand how galaxies grow, transform, and sometimes leave stars behind in their wake.

Source: George