Long strands of glowing gas stretch behind a distant galaxy, dotted with pockets of newborn stars. The shape looks almost biological, like tentacles drifting through water. Yet this structure formed in one of the most extreme environments in the universe.<\/p>\n
Astronomers have identified what may be the most distant known \u201cjellyfish galaxy,\u201d a system caught in the act of being stripped apart by its surroundings about 8.5 billion years ago. The object, cataloged as COSMOS2020-635829, sits at a redshift of 1.156 and appears to be plunging through a dense cluster environment that is tearing gas from its disk.<\/p>\n
The discovery comes from a team led by researchers at the University of Waterloo<\/a>, who analyzed deep observations from the James Webb Space Telescope alongside spectroscopy from the Gemini Observatory. Their findings suggest that galaxy clusters were already harsh environments much earlier in cosmic history than many astronomers expected.<\/p>\n \u201cWe were looking through a large amount of data from this well-studied region in the sky with the hopes of spotting jellyfish galaxies that haven\u2019t been studied before,\u201d said Dr. Ian Roberts, a Banting Postdoctoral Fellow at the Waterloo Centre for Astrophysics. \u201cEarly on in our search of the JWST data, we spotted a distant, undocumented jellyfish galaxy that sparked immediate interest.\u201d<\/p>\n Thumbnail images of COSMOS2020-635829 for the four JWST filters used in this work. The red\u2013green\u2013blue image on the right is a combination of JWST F444W (red channel), F277W (green channel), and F115W + F150W (blue channel). The dashed circles mark the four extraplanar sources that are identified in the tail of COSMOS2020-635829. (CREDIT: The Astrophysical Journal) A violent environment in the young universe<\/p>\n Galaxies do not evolve in isolation. For decades, astronomers have known that systems living in crowded clusters tend to stop forming stars sooner than galaxies drifting alone in space. That process, called environmental quenching, is often linked to the loss of cold gas, the raw material needed to build new stars.<\/p>\n One suspected mechanism is ram-pressure stripping. As a galaxy moves through hot gas that fills a cluster, pressure acts like a headwind, pushing the galaxy\u2019s own gas backward and sometimes completely out of the system. Nearby examples often show dramatic one-sided tails, which inspired the nickname jellyfish galaxies.<\/p>\n Clear evidence of this process at large cosmic distances has been scarce. Detecting faint gas structures<\/a> requires both sensitivity and sharp resolution, conditions that only recently became possible with instruments like JWST.<\/p>\n COSMOS2020-635829 appears to provide that missing piece.<\/p>\n The galaxy itself has a stellar mass of roughly 10\u00b9\u2070 solar masses and an intense star-formation rate near 100 solar masses per year. High-resolution imaging shows a fairly normal disk, but bright blue knots trail to one side. These knots contain very young stars, likely formed from gas that has already been stripped from the main galaxy.<\/p>\n A tail confirmed in gas<\/p>\n To test whether the visual features truly came from ram-pressure stripping, the researchers turned to spectroscopy using the Gemini Multi-Object Spectrograph integral field unit. They focused on the [O II] emission line, a common tracer of ionized gas.<\/p>\n Large-scale structure around COSMOS2020-635829. The large star marks the position of COSMOS2020-635829. (CREDIT: The Astrophysical Journal) <\/p>\n The results revealed gas emission extending about 20 kiloparsecs beyond the galaxy\u2019s disk along the same direction as the visible tail. A smooth velocity gradient connects this gas to the rotating galaxy<\/a>, confirming that the material belongs to the system rather than a background source.<\/p>\n Maps of the emission show a diffuse plume trailing south of the galaxy, with brightness steadily declining away from the disk. The structure lacks the ring-like pattern expected from certain collision scenarios, which strengthens the case for ram-pressure stripping.<\/p>\n The team still considered alternative explanations, including the possibility that the galaxy could be a collisional ring system. However, several observations argue against that idea. Ionized gas peaks at the galaxy center rather than in a ring, and the morphology does not resemble classic ring galaxies such as Hoag\u2019s Object. Researchers concluded that ram pressure remains the most plausible driver, though they cannot fully rule out other processes.<\/p>\n Star formation outside the galaxy<\/p>\n Four compact regions embedded in the tail show particularly strong blue light. Using spectral energy distribution modeling with data from JWST, the Hubble Space Telescope<\/a>, and Subaru, the team estimated their properties.<\/p>\n Each knot contains about 10\u2078 solar masses of stars and has been forming stars within the past 100 million years. Star-formation rates are roughly a few tenths of a solar mass per year. Together, the tail regions account for about 1 percent of the main galaxy\u2019s stellar mass and star-formation activity.<\/p>\n These numbers are broadly consistent with massive star-forming knots seen in nearby jellyfish galaxies once cosmic evolution is considered. Star formation rates in galaxies were higher at redshift 1 than they are today, which may explain the somewhat elevated values.<\/p>\n Color profile for the main disk of COSMOS2020-635829 measured in circular annuli from the galaxy center to the edge of the disk. (CREDIT: The Astrophysical Journal) <\/p>\n The fate of these stripped regions remains uncertain. Some simulations suggest that stars formed within about 20 kiloparsecs of a galaxy may eventually fall back into the disk. Other scenarios allow them to become independent objects or dissolve into diffuse intracluster light, the faint glow that fills galaxy clusters.<\/p>\n Rethinking cluster evolution<\/p>\n Perhaps the most significant implication involves the environment itself. Astronomers previously thought that clusters at this epoch were still assembling and might not yet have conditions strong enough to strip galaxies efficiently.<\/p>\n Instead, the observations indicate that dense intracluster gas was already capable of exerting substantial ram pressure.<\/p>\n \u201cThe first is that cluster environments were already harsh enough to strip galaxies, and the second is that galaxy clusters may strongly alter galaxy properties earlier than expected,\u201d Roberts said. \u201cAnother is that all the challenges listed might have played a part in building the large population of dead galaxies we see in galaxy clusters<\/a> today. This data provides us with rare insight into how galaxies were transformed in the early universe.\u201d<\/p>\n Estimates based on X-ray observations of nearby overdensities suggest intracluster gas densities around 10\u207b\u00b2\u2077 to 10\u207b\u00b2\u2076 grams per cubic centimeter. Combined with galaxy velocities of several hundred to a thousand kilometers per second, those values produce ram-pressure forces comparable to what astronomers measure in nearby clusters.<\/p>\n If confirmed, this would push the known operation of ram-pressure stripping back to earlier cosmic times, closer to the period sometimes called Cosmic Noon, when star formation across the universe peaked.<\/p>\n A rare window into early quenching<\/p>\n Evidence for stripping at redshifts above 1 has been limited to a handful of candidate systems. Some molecular gas tails have been reported in a protocluster at redshift 2.51, but direct optical emission signatures remain rare.<\/p>\n