2019A&A...631A.114B

Multi-Unit Spectroscopic Explorer (MUSE) observations of the cluster of galaxies CGr32 (M₂₀₀~=2x10¹⁴M_☉) at z=0.73 reveal the presence of two massive star-forming galaxies with extended tails of diffuse gas detected in the [OII]λλ3727-3729Å emission-line doublet. The tails, which have a cometary shape with a typical surface brightness of a few 10^–18erg/s/cm²/arcsec², extend up to ~=100kpc (projected distance) from the galaxy discs, and are not associated with any stellar component. All this observational evidence suggests that the gas was removed during a ram-pressure stripping event. This observation is thus the first evidence that dynamical interactions with the intracluster medium were active when the Universe was only half its present age. The density of the gas derived using the observed [OII]λ3729/[OII]λ3726 line ratio implies a very short recombination time, suggesting that a source of ionisation is necessary to keep the gas ionised within the tail.