Resolved Spectroscopy of a Gravitationally Lensed L*Lyman Break Galaxy at z~5

We exploit the gravitational potential of a massive, rich cluster at z = 0.77 to study the internal properties of a gravitationally lensed galaxy at z = 4.88. Using high-resolution Hubble Space Telescope imaging together with optical (VIMOS) and near-infrared (SINFONI) integral field spectroscopy, we have studied the rest-frame ultraviolet and optical properties of the lensed galaxy seen through the cluster RCS0224-002. Using a detailed gravitational lens model of the cluster, we reconstruct the source-frame morphology on 200 pc scales and find an similar to L* Lyman-break galaxy with an intrinsic size of only 2.0 x 0.8 kpc, a velocity gradient of less than or similar to 60 km s(-1) and an implied dynamical mass of 1.0 x 10(10) M-circle dot within 2 kpc. We infer an integrated star formation rate of just 12 2 M-circle dot yr(-1) from the intrinsic [O II]lambda 3727 emission-line flux. The Ly alpha emission appears redshifted by +200 +/- 40 km s(-1) with respect to the [O II] emission. The Ly alpha is also significantly more extended than the nebular emission, extending over 11.9 x 2.4 kpc. Over this area, the Ly alpha centroid varies by less than 10 km s(-1). We model the asymmetric Lya emission with an underlying Gaussian profile with an absorber in the blue wing and find that the underlying Ly alpha emission-line centroid is in excellent agreement with the [O II] emission line redshift. By examining the spatially resolved structure of the [O II] and Ly alpha emission lines, we investigate the nature of this system. The model for local starburst galaxies suggested by Mas-Hesse et al. provides a good description of our data, and suggests that the galaxy is surrounded by a galactic-scale bipolar outflow which has recently bursted out of the system. The outflow, which appears to be currently located greater than or similar to 30 kpc from the galaxy, is escaping at a speed of upto similar to 500 km s(-1). 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 1 Mpc (comoving) before eventually stalling.