Identification of a gravitationally lensed z=2.515 star-forming galaxy

We discuss the optical spectrum of a multiply imaged are resolved by the Hubble Space Telescope in the z = 0.175 cluster A2218. The spectrum, obtained with LDSS-2 on the 4.2-m William Herschel Telescope, reveals the source to be a galaxy at a redshift z = 2.515, in excellent agreement with the value predicted by Kneib et al. on the basis of their inversion of a highly constrained mass model for the lensing cluster. The source is extremely blue in its optical-infrared colours, consistent with active star formation, and the spectrum reveals absorption lines characteristic of a young stellar population. Of particular significance is the absence of Ly alpha emission but the presence of a broad Ly alpha absorption. The spectrum is similar to that of other, much fainter, galaxies found at high redshift by various techniques, and illustrates the important role that lensing can play in detailed studies of the properties of distant galaxies.