AAS 195th Meeting, January 2000
Session 9. Evolution of Galaxies, Galaxy Surveys
Display, Wednesday, January 12, 2000, 9:20am-6:30pm, Grand Hall

## [9.12] The Evolution of Far-UV Emission in Elliptical Galaxies

T. M. Brown, C. W. Bowers, R. A. Kimble (NASA/GSFC), H. C. Ferguson (STScI)

The restframe UV-to-optical flux ratio, characterizing 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 Gyr. In order to trace the evolution of the UV upturn as a function of redshift, we have used the far-UV camera on the Space Telescope Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our 25 \arcsec \times 25 \arcsec field includes four bright elliptical galaxies, spectroscopically confirmed to be passively evolving cluster members. The weak UV emission from the galaxies in our image demonstrates that the UV upturn is very weak at a lookback time ~5.6~Gyr earlier than our own, as compared to measurements of the UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the first with sufficient depth to demonstrate the fading of the UV upturn expected at moderate redshifts. We discuss these observations and the implications for the formation history of galaxies.