Observing Ram Pressure at Work in Intermediate Redshift Clusters with MUSE: The Case of Abell 2744 and Abell 370

Ram pressure stripping has been proven to be effective in shaping galaxy properties in dense environments at low redshift. The availability of Multi Unit Spectroscopic Explorer (MUSE) observations of a sample of distant ( z ∼ 0.3–0.5) clusters has allowed one to search for galaxies subject to this phenomenon at significant lookback times. In this paper we describe how we discovered and characterized 13 ram-pressure-stripped galaxies in the central regions of two intermediate redshift ( z ∼ 0.3–0.4) clusters, A2744 and A370, using the MUSE spectrograph. Emission-line properties as well as stellar features have been analyzed to infer the presence of this gas-only stripping mechanism, that produces spectacular ionized gas tails (H α and even more astonishing [O ii ](3727, 3729)) departing from the main galaxy body. The inner regions of these two clusters reveal the predominance of such galaxies among blue star-forming cluster members, suggesting that ram pressure stripping was even more effective at intermediate redshift than in today’s universe. Interestingly, the resolved [O ii ]/H α line ratio in the stripped tails is exceptionally high compared to that in the disks of these galaxies, (which is comparable to that in normal low- z galaxies), suggesting lower gas densities and/or an interaction with the hot surrounding intracluster medium.