A gravitationally-boosted MUSE survey for emission-line galaxies at z>~5
  behind the massive cluster RCS 0224
  We present a VLT/MUSE survey of lensed high-redshift galaxies behind the
z=0.77 cluster RCS0224-0002. We study the detailed internal properties of a
highly magnified ({\mu}~29) z=4.88 galaxy seen through the cluster. We detect
wide-spread nebular CIV{\lambda}{\lambda}1548,1551{\AA} emission from this
galaxy as well as a bright Ly{\alpha} halo with a spatially-uniform wind and
absorption profile across 12 kpc in the image plane. Blueshifted high- and
low-ionisation interstellar absorption indicate the presence of a high-velocity
outflow ({\Delta}v~300 km/s) from the galaxy. Unlike similar observations of
galaxies at z=2-3, the Ly{\alpha} 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 Ly{\alpha} photons. We also search for
high-redshift Ly{\alpha} emitters and identify 14 candidates between z=4.8-6.6,
including an over-density at z=4.88, of which only one has a detected
counterpart in HST/ACS+WFC3 imaging.
