Using the Bullet Cluster as a Gravitational Telescope to Study z~7 Lyman   Break Galaxies
  We use imaging obtained with the Hubble Space Telescope Wide Field Camera 3 to search for z_850 dropouts at z~7 and J_110 dropouts at z~9 lensed by the Bullet Cluster. In total we find 10 z_850 dropouts in our 8.27 arcmin^2 field. Using magnification maps from a combined weak and strong lensing mass reconstruction of the Bullet Cluster and correcting for estimated completeness levels, we calculate the surface density and luminosity function of our z_850 dropouts as a function of intrinsic (accounting for magnification) magnitude. We find results consistent with published blank field surveys, despite using much shallower data, and demonstrate the effectiveness of cluster surveys in the search for z~7 galaxies. 
Nicholas Hall, Marusa Bradac, Anthony H. Gonzalez, Tommaso Treu,   Douglas Clowe, Christine Jones, Massimo Stiavelli, Dennis...
astro-ph.CO