Hubble Space Telescope observations of the lensing cluster Abell 2218

We present a striking new Hubble Space Telescope (HST) observation of the rich cluster Abell 2218 taken with the Wide Field Planetary Camera 2. HST's restored image quality reveals a sizable number of gravitationally lensed features in this cluster, significantly more than had been identified by using ground-based telescopes. The brightest area are resolved by HST and show internal features that enable us to identify multiply imaged examples, confirming and improving the mass models of the cluster determined from ground-based observations. Although weak lensing has been detected statistically in this and other clusters from ground-based data, the superlative resolution of HST enables us to individually identify weakly distorted images more reliably than hitherto, with important consequences for their redshift determination. Using an improved mass model for the cluster calibrated with available spectroscopy for the brightest arcs, we demonstrate how inversion of the lensing model can be used to yield the redshift distribution of similar to 80 faint arclets to R similar or equal to 25. We present a new formalism for estimating the uncertainties in this inversion method and review prospects for interpreting our results and verifying the predicted redshifts.