16876

Studies of the most highly ionized galaxies in the universe offer insights into the complex interplay between feedback comma shocks comma and radiative cooling comma driven by extreme star formation or accreting supermassive black holes. We here propose eight orbits of broad and narrowband imaging of J1356-2313 comma a puzzling comma newly discovered gravitationally lensed source at z=2.6 with the highly unusual combination of extreme high ionization UV line emission from O VI comma N V comma He II and C IV coupled with damped Lya absorption. Integral field spectroscopy of the kinematically narrow O VI emission reveals that it is gravitationally lensed into an arc comma indicating a spatially extended source larger than an AGN narrow-line region further comma the line is kinematically inconsistent with a quasar outflow. The very high ionization emission lines may instead be a signature of catastrophic radiative cooling comma which may suppress feedback in the most extreme starbursts. With the proposed broadband imaging comma we will construct a lensing model and determine the intrinsic luminosity of the source and the structure of the emitting region(s). Modeling of the spectral energy distribution will offer further constraints on the nature of the ionizing source. We also propose narrowband imaging of the extremely strong and spatially extended O VI emission comma which is fortuitously redshifted into the F373N filter. The spatial resolution of HST in combination with the magnification of gravitational lensing will provide an unprecedented map of extremely high ionization line emission in the distant universe comma allowing tests of the emission mechanisms discussed above.