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

We discuss the optical spectrum of a multiply-imaged arc resolved by HST inthe $z$=0.175 cluster A2218. The spectrum, obtained with LDSS-2 on the 4.2mWilliam 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.(1996) on the basis of their inversion of a highly-constrained mass model forthe lensing cluster. The source is extremely blue in its optical-infraredcolours, consistent with active star formation, and the spectrum revealsabsorption lines characteristic of a young stellar population. Of particularsignificance is the absence of Lyman-$\alpha$ emission but the presence of abroad Lyman-$\alpha$ absorption. The spectrum is similar to that of other, muchfainter, galaxies found at high redshift by various techniques and illustratesthe important role that lensing can play in detailed studies of the propertiesof distant galaxies.