By combining HST imaging with optical (VIMOS) and near-infrared (SINFONI) integral field spectroscopy we exploit the gravitational potential of a massive, rich cluster at z=0.9 to study the internal properties of a gravitationally lensed galaxy at z=4.88. Using a detailed gravitational lens model of the cluster RCS0224-002 we reconstruct the source-frame morphology of the lensed galaxy on 200pc scales and find an ~L* Lyman-break galaxy with an intrinsic size of only 2.0x0.8kpc, a velocity gradient of <60km/s and an implied dynamical mass of 1.0x10^10Mo within 2kpc. We infer an integrated star-formation rate of just 12+/-2Mo/yr from the intrinsic [OII] emission line flux. The Ly-alpha emission appears redshifted by +200+/-40km/s with respect to the [OII] emission. The Ly-alpha is also significantly more extended than the nebular emission, extending over 11.9x2.4kpc. Over this area, the Ly-alpha centroid varies by less than 10km/s. By examining the spatially resolved structure of the [OII] and asymmetric Ly-alpha emission lines we investigate the nature of this system. The model for local starburst galaxies suggested by Mass-Hesse et al. (2003) provides a good description of our data, and suggests that the galaxy is surrounded by a galactic-scale bi-polar outflow which has recently burst out of the system. The outflow, which appears to be currently located >30kpc from the galaxy, is escaping at a speed of upto ~500km/s. Although the mass of the outflow is uncertain, the geometry and velocity of the outflow suggests that the ejected material is travelling far faster than escape velocity and will travel more than 1Mpc (comoving) before eventually stalling.
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