Session 63 - Galaxy Evolution II.
Oral session, Tuesday, January 16
Corte Real, Hilton

## [63.06] The Photometric, Spectroscopic and Morphological Evolution of Starburstin g Galaxies in Distant Clusters

A. J. Barger, A. Aragón-Salamanca, R. S. Ellis (Cambridge), W. J. Couch (UNSW), I. Smail (Carnegie), R. M. Sharples (Durham)

From WFPC images of 3 rich clusters at a redshift of z=0.31, we have found that the unexpectedly high fraction of blue galaxies present in distant clusters (as compared to nearby clusters, the Butcher-Oemler Effect'') are predominantly disk galaxies, many exhibiting morphological signatures of recent merger activity or tidal interactions. Red galaxies with strong Balmer absorption lines appear mainly to be regular spheroidals. The WFPC images therefore strongly suggest that galaxy-galaxy mergers play a large role in causing starbursts in distant cluster galaxies. If one such underlying physical process is indeed responsible for the abnormal star-forming activity seen in distant cluster galaxies, then the various anomalous galaxies observed might represent different stages of a single starburst cycle. A critical breakthrough in understanding is now possible through the use of evolutionary modelling, based on a stellar population synthesis code, to reproduce the frequency of galaxies observed in the various evolutionary stages. Using the number density of spectroscopically-confirmed members on the EW(H\delta) versus B-R plane and for a larger K'-limited sample on the U-I versus I-K' plane from infrared images, we demonstrate that about 30 per cent of all distant cluster galaxies must undergo secondary bursts of star formation within \sim 2 Gyr prior to observation. The model fits also suggest that the bursts are typically short-lived and convert 10-20 per cent of the final stellar mass into stars. New data from even higher redshift clusters (0.4 Program listing for Tuesday