Far-ultraviolet emission from elliptical galaxies at z=0.55

The rest-frame W-to-optical flux ratio, which characterizes the "UV upturn" phenomenon, is potentially the most sensitive tracer of age in elliptical galaxies; models predict that it may change by orders of magnitude over the course of a few gigayears. In order to trace the evolution of the UV uptur n as a function of redshift, we have used the far-UV camera on the Space Te lescope Imaging Spectrograph to image the galaxy cluster CL 0016+16 at,z = 0.55. Our 25" x 25" field includes four bright elliptical galaxies, spectro scopically confirmed to be passively evolving cluster members. The weak UV emission from the galaxies in our image demonstrates that the UV upturn is weaker at a look-back time similar to 5.6 Gyr earlier than our own, as comp ared to measurements of the UV upturn in cluster E and SO galaxies at z = 0 and z = 0.375. These images are the first with sufficient depth to demonst rate the fading of the UV upturn expected at moderate redshifts. We discuss these observations and the implications for the formation history of galax ies.