Evidence for ram-pressure stripping in a cluster of galaxies at z=0.7

Multi-Unit Spectroscopic Explorer (MUSE) observations of the cluster of galaxies CGr32 (M-200 similar or equal to 2 x 10(14) M-circle dot) at z = 0:73 reveal the presence of two massive star-forming galaxies with extended tails of di ffuse gas detected in the [OII]lambda lambda 3727-3729 angstrom emissionline doublet. The tails, which have a cometary shape with a typical surface brightness of a few 10(-18) erg s(-1) cm(-2) arcsec 2, extend up to similar or equal to 100 kpc (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]lambda 3729/[OII]lambda 3726 line ratio implies a very short recombination time, suggesting that a source of ionisation is necessary to keep the gas ionised within the tail.