Discovery of a luminous giant arc in a high redshift cluster of galaxies

We report the discovery of a bright, R = 19.5, arc-like feature associated with a rich cluster of galaxies at a redshift of z = 0.56, found in the course of a programme of optical identification of powerful ultra-steep spectrum radio sources at high redshifts. The redshift of this nearly straight arc, z = 1 .116, implies that the feature is the gravitationally lensed image of a background galaxy. The morphology of the arc, its emission line spectrum and blue colour, and its extremely high surface brightness indicate that the lensed galaxy is likely to be a luminous starburst galaxy, probably a merger of two late type systems. The new arc is the brightest and one of the most distant giant arcs discovered to date, and the only spectroscopically confirmed arc known to be lensed by a cluster at z > 0.5