EARLY-TYPE POST-STARBURST GALAXIES IN THE COMA CLUSTER

the butcheroemler effect (butcher and oemler 1978, 1984; boeffect in what follows) is the evidence for the existence of a different population of galaxies in distant clusters that is not found in nearby ones. this population consists of a mix of starburst and poststarburst galaxies, both blue spirals and red earlytype galaxies. the physical process(es) responsible for this effect, i.e. a rapid evolution of cluster galaxies, has (have) not been found yet, although many have been proposed (e.g. oemler 1992). the difficulty lies in the fact that these galaxies are quite distant (redshift of about 0.3). recently, however, caldwell et al. (1993) have identified a lowredshift analogue of the boeffect in a sample of 17 earlytype galaxies in the coma cluster. these galaxies have an enhanced balmer absorption, which is abnormal for their red colours, plus (in some cases) emissionlines. their spectrum is indicative of a poststarburst phase that would have occurred 1 gyr ago in an earlytype galaxy, or of a truncated stellar formation in a latetype one. most important, their spectra are very similar to those seen in a distant cluster of galaxies which shows evidence of the boeffect, so that studying the properties of these easyaccess relatively nearby galaxies will finally explain what is the cause of the boeffect. ir observations with iso will be extremely useful for characterizing the poststarburst properties of these galaxies, thanks to the high sensitivity achievable and the large wavelength range that can be covered. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]