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Spectroscopic Confirmation of Redshifts Predicted by Gravitational Lensing

We present deep spectroscopic measurements of 18 distant field galaxies identified as gravitationally-lensed arcs in a Hubble Space Telescope image of the cluster Abell 2218. Redshifts of these objects were predicted by Kneib et al. (1996) using a lensing analysis constrained by the properties of two bright arcs of known redshift and other multiply-imaged sources. The new spectroscopic identifications were obtained using long exposures with the LDSS-2 spectrograph on the William Herschel Telescope and demonstrate the capability of that instrument to new limits, $R\simeq$24; the lensing magnification implies true source magnitudes as faint as $R\simeq$25. Statistically, our measured redshifts are in excellent agreement with those predicted from Kneib et al.'s lensing analysis which gives considerable support to the redshift distribution derived by the lensing inversion method for the more numerous and fainter arclets extending to $R\simeq 25.5$. We explore the remaining uncertainties arising from both the mass distribution in the central regions of Abell 2218 and the inversion method itself, and conclude that the mean redshift of the faint field population at $R\simeq$25.5 ($B\sim 26$-27) is low, $<z>$=0.8-1. We discuss this result in the context of redshift distributions estimated from multi-colour photometry. Although such comparisons are not straightforward, we suggest that photometric techniques may achieve a reasonable level of agreement particularly when they include near-infrared photometry with discriminatory capabilities in the $1<z<$2 range.