Program: GN-2011A-DD-5

We request Director's Discretionary Time to obtain a spectroscopic redshift for a strong gravitational arc behind a galaxy cluster that we recently confirmed to lie at z=1.75. This system is by far the highest redshift cluster behind which a gravitational arc has been detected, and the presence of a massive galaxy cluster (potentially M>1e15) at this redshift poses a significant challenge for vanilla LCDM structure formation. A spectroscopic redshift for this arc would lead to rapid submission of a high-profile paper based upon Gemini observations. The observations are high-risk (see the technical justification), but high-reward. We are requesting DDT observations because of the potentially large impact of these observations and the fact that we are nearing the end of the observing season for this field. We have not previously applied through the regular proposal cycle because the arc was only discovered in late April upon examination of HST data for the cluster, and the redshift of the cluster was confirmed at roughly the same time.