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

Abstract

MUSE observations of the cluster of galaxies CGr32 ($M_{200}$ $\simeq$ 2 $\times$ 10$^{14}$ M$_{\odot}$) at $z$ = 0.73 reveal the presence of two massive star forming galaxies with extended tails of diffuse gas detected in the [OII]$\lambda\lambda$3727-3729 A emission-line doublet. The tails, which have a cometary shape with a typical surface brightness of a few 10$^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$, extend up to $\simeq$ 100 kpc (projected distance) from the galaxy discs and are not associated to any stellar component. All this observational evidence suggests that the gas has been 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 had only half of 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.