Astronomers have spotted many “red and dead” galaxies in the early universe. These are massive systems that stopped forming stars surprisingly early in cosmic history. Now, they may have found evidence of one in the act of becoming dead: a massive galaxy being stripped of its star-forming gas just 1.4 billion years after the Big Bang. The clues behind why it lost its star-forming material are detailed in a paper posted to the arXiv preprint server on June 16.
Comet-like galaxy
SPT2349–56 is an emerging galaxy cluster, or “protocluster,” containing about 30 star-forming galaxies within a region 100 kiloparsecs wide. Among its members, C26 is particularly interesting because of its unusual shape. It has a head and a tail like a comet. It also has a dense, bright region called the “knot,” embedded within the tail. It was first detected in ALMA images.
In this new study, using observations from the Hubble Space Telescope and the James Webb Space Telescope, the team led by Dazhi Zhou of the University of British Columbia studied this galaxy’s head, tail and knot to estimate its mass and star-forming properties.
Interestingly, the tail seems to house a younger stellar population, as it is detected in UV light. The stellar head has a mass of around 22 billion solar masses, and the tail, including the knot, has a mass of around 6 billion solar masses.
The specific star-formation rate of the head is lower than expected for a typical star-forming galaxy, whereas the tail and knot are consistent with expectations.
Calculating the amount of cold gas in the galaxy available for the formation of new stars, the team found that while tens of billions of solar masses of gas are present, more than half of it is not even inside the galaxy. This displaced gas has been pulled out into the long tail trailing behind it, and it appears diffuse and calm—not really dense and turbulent gas suitable for star formation.
Merger or stripping?
There are two ways galaxies typically lose gas like this: tidal interaction or merger, where gravity from another galaxy pulls gas away, and ram-pressure stripping, which occurs when the galaxy plows through a hot, dense gas medium and that resistance physically strips gas off it.
The team rules out a merger because the only candidate for a merging companion—a bright knot embedded in the tail—is too low-mass to gravitationally rip away that much gas.
Instead, several clues point to ram-pressure stripping. The stripped gas moves smoothly and continuously, rather than as a torn-off fragment. It’s calm and diffuse, and despite holding onto a massive gas supply, the galaxy’s star formation remains surprisingly low—unlike the starburst expected from a merger. The tail also happens to point toward the center of the cluster, consistent with gas trailing behind a galaxy as it moves through hot intracluster gas.
“Together with the tail alignment and the independently detected hot ICM in SPT2349−56, these observations therefore favor ram-pressure stripping over tidal interaction as the main mechanism shaping C26,” the team writes in the paper.
This kind of dramatic stripping is what creates “jellyfish galaxies” that flaunt tentacle-like tails of gas. But it’s normally assumed this needs a mature, well-developed cluster with a hot, dense intracluster medium to work efficiently. SPT2349−56 is still a young cluster in the process of forming, very early in the universe’s history.
Red and dead
Ram-pressure stripping effectively starves the galaxy of fuel. Without gas, star formation eventually shuts down in a process called “quenching.” The team suggests that C26 may be caught mid-transformation, where a massive, still-star-forming galaxy has already lost most of its gas and is on its way to becoming a dead, non-star-forming galaxy.
“C26 may capture an intermediate stage between these two regimes, in which most of the cold-gas reservoir has already been removed by the external environment, while the stellar head is not yet fully quenched,” they explain.
As a result, this connects to a broader puzzle in astronomy about why astronomers find quiet, mature-looking galaxies surprisingly early in cosmic history. Interestingly, other galaxies in this same cluster core also show signs of being gas-poor, suggesting this stripping process might be actively reshaping the whole protocluster, not just C26.
The team notes that follow-up studies with high-resolution ALMA and JWST data would give more context on the interaction between the hot gas within a galaxy cluster and the star-forming gas of cluster members.
Written for you by our author Shreejaya Karantha, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
If this reporting matters to you, please consider a donation (especially monthly). You’ll get an ad-free account as a thank-you.
Publication details
Dazhi Zhou et al, An extreme ram-pressure stripping event in a protocluster at redshift 4.3, arXiv (2026). DOI: 10.48550/arxiv.2606.18229
Journal information:
arXiv
© 2026 Science X Network
Citation:
Astronomers may have caught an early galaxy in the process of dying (2026, July 4)
retrieved 4 July 2026
from https://phys.org/news/2026-06-astronomers-caught-early-galaxy-dying.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.