A gravitationally boosted MUSE survey for emission-line galaxies at z $\gtrsim$ 5 behind the massive cluster RCS 0224

International audienceWe present a Very Large Telescope/Multi Unit Spectroscopic Explorer (MUSE) survey of lensed high-redshift galaxies behind the z = 0.77 cluster RCS 0224−0002. We study the detailed internal properties of a highly magnified (μ ∼ 29) z = 4.88 galaxy seen through the cluster. We detect widespread nebular C iv λλ1548,1551 Å emission from this galaxy as well as a bright Lyα halo with a spatially uniform wind and absorption profile across 12 kpc in the image plane. Blueshifted high- and low-ionization interstellar absorption indicate the presence of a high-velocity outflow (Δv ∼ 300 km s^− 1) from the galaxy. Unlike similar observations of galaxies at z ∼ 2 − 3, the Lyα emission from the halo emerges close to the systemic velocity – an order of magnitude lower in velocity offset than predicted in ‘shell’-like outflow models. To explain these observations, we favour a model of an outflow with a strong velocity gradient, which changes the effective column density seen by the Lyα photons. We also search for high-redshift Lyα emitters and identify 14 candidates between z = 4.8 − 6.6, including an overdensity at z = 4.88, of which only one has a detected counterpart in Hubble Space Telescope/Advanced Camera for Surveys+Wide Field Camera 3 imaging