An international team led by Prof. Tao Wang of Nanjing University has captured, for the first time, multiple galaxies undergoing ram-pressure stripping in a young galaxy cluster at z = 2.51, when the Universe is only ~3 billion years old. This discovery brings forward the observational record of such environmental processes in galaxy clusters by almost 5 billion years in cosmic history. The result has been published online in Nature Astronomy on 22 June 2026 under the title "Ram-pressure stripping caught in action in a young cluster at z = 2.51".
Figure 1. The most distant ram-pressure stripping event ever recorded, captured in a young galaxy cluster at z = 2.51 (11 billion years ago), captured by an international team led by astronomers in Nanjing University using ALMA and JWST. (Ram-pressure stripping: cold gas in a galaxy is pushed out by the pressure of the hot intracluster medium as the galaxy moves at high speed through a galaxy cluster.)
In the nearby universe, galaxy clusters are dominated by massive red galaxies that have largely ceased forming new stars. While these galaxies built most of their stars during the early universe, exactly when and through what mechanisms they stopped forming new stars remains one of the central open questions in galaxy evolution. In particular, whether the dense environments of early-universe galaxy clusters had already begun to affect the gas supply of their member galaxies and drive them toward "quenching" has long lacked direct observational evidence.
The research team used the Atacama Large Millimeter/submillimeter Array (ALMA) and the James Webb Space Telescope (JWST) to conduct deep, multi-wavelength studies of the galaxy cluster CLJ1001 at redshift z = 2.51. ALMA traced cold molecular gas via carbon monoxide (CO) emission lines, while JWST mapped stellar distributions in the near-infrared. The results show that five galaxies in this cluster exhibit pronounced asymmetric, one-sided gas tails extending well beyond their stellar bodies — as though blown out of the galaxy by a head-on "cosmic hot wind." More critically, while the cold gas is stretched and distorted, the stellar distributions remain regular, with no signatures of recent mergers or tidal interactions. This indicates that the gas is not being gravitationally pulled away by neighboring galaxies, but is being rapidly removed by the hot gaseous medium of the cluster itself — the canonical process known as ram-pressure stripping.
"Ram-pressure stripping happens when a galaxy moves through the hot gas of a galaxy cluster and the head-on high-pressure hot wind blows away its own cold gas," explained Professor Tao Wang, the corresponding author of the study. "We commonly see this phenomenon in the nearby universe, but we have never observed it in such an early galaxy cluster"
Even more surprising, the galaxies caught in the act of losing their cold gas have not immediately stopped forming stars — on the contrary, they exhibit exceptionally active star formation, with star formation efficiencies approximately twice those of undisturbed cluster galaxies. The research team proposes that ram-pressure stripping, while ejecting gas, also stirs and compresses the remaining cold gas in the galaxy, triggering a short but intense episode of star formation before the gas is entirely removed. In other words, these galaxies may be experiencing a final burst of star formation — a last blaze — on their way toward quenching.
This study demonstrates that the suppression of star formation in massive galaxies by cluster environments cannot be overlooked, and that such suppression can begin before clusters are fully assembled. As early as ~11 billion years ago, the hot gaseous medium in early-universe galaxy clusters was already capable of significantly affecting the gas supply of member galaxies and driving them toward "quenching." By extending direct observations of ram-pressure stripping to the highest redshift currently known, this research adds a critical frame from the cosmic noon to one of the central questions in extragalactic astronomy: the origin of massive red galaxies in clusters.
Mr. Ke Xu (PhD student, Nanjing University) is the first author; Prof. Tao Wang (Nanjing University) is the corresponding author. Collaborators include Prof. Zhiyu Zhang and Dr. Luwenjia Zhou (Nanjing University), Dr. Daizhong Liu (Purple Mountain Observatory), Prof. Renyue Cen (Zhejiang University), and Prof. Emanuele Daddi and Prof. David Elbaz (Université Paris-Saclay), among others. This work was supported by the National Natural Science Foundation of China, the Natural Science Foundation of Jiangsu Province, the Nanjing University "789" Science and Technology Initiative, and the National Key R&D Program of China.
Article Link: https://www.nature.com/articles/s41550-026-02892-9