Program: GN-2012A-Q-44

We propose to obtain the redshift of the strongly lensed arc in Abell 781D, the western component in the complex of clusters that is Abell 781. This will allow us to measure the mass of the central region of this peculiar cluster. Although both the X-ray emission and the velocity dispersion of galaxies in the cluster suggest a significant mass, three weak lensing measurements of the cluster, made with different telescopes and different source depths, all consistently show a much lower signal. It is essential to determine the origin of systematic errors and biases in weak lensing mass estimates for the successful calibration of mass-observables in future cosmological surveys. To help unravel the mystery of the mass discrepancy in Abell 781D, we need to have an accurate mass scale measurement of the cluster. Fortunately there is a large blue strong lensing arc near the central galaxy of the cluster, however an optical spectrum of the arc revealed no identifiable emission line features. We therefore request GNIRS spectroscopy to obtain the redshift of the arc to complete the mass calibration of the cluster. It will then be possible to measure the mass density of the central region via strong lensing analysis.