2003ApJ...584L..69B

We present far-UV images of the rich galaxy cluster ZwCl 1358.1+6245, taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). When combined with archival HST observations, our data provide a measurement of the UV-to-optical flux ratio in eight early-type galaxies at z=0.33. Because the UV flux originates in a population of evolved, hot, horizontal-branch (HB) stars, this ratio is potentially one of the most sensitive tracers of age in old populations–it is expected to fade rapidly with look-back time. We find that the UV emission in these galaxies, at a look-back time of 3.9 Gyr, is significantly weaker than it is in the current epoch, yet it is similar to that in galaxies at a look-back time of 5.6 Gyr. Taken at face value, these measurements imply different formation epochs for the massive elliptical galaxies in these clusters, but an alternative explanation is a ``floor'' in the UV emission due to a dispersion in the parameters that govern HB morphology.