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