2007ApJ...671.1182I

The long exposure times of the Hubble Ultra Deep Field (UDF) together with empirically derived position-dependent point-spread functions have enabled us to measure two distinct nonlinear shape distortions of galaxies. We have previously argued that nonlinear lensing from small (10⁹-10¹⁰ M_☉) dense objects in the foreground might be observable as localized concentrations of curved background galaxy shapes. Such clumping was evident in our analysis of the Hubble Deep Field-North (HDF-N). We have repeated this analysis in the UDF with an enhanced statistical procedure, with more sources per square arcminute, and with better resolved background sources. We have found an even stronger clumping signal with maximum clumping at the same angular scale (12.5" radius) as found in the HDF-N. Furthermore, we have found an important correlation between the three coefficients we have measured that must be founded on features of the background galaxy shapes themselves. Galaxies maximally departing from this correlation, which we call d-break galaxies, should also indicate lensing, even linear lensing. Indeed, we find that d-break galaxies are also concentrated, but on smaller angular scales (about 6" radius) than the curved galaxies.