Detectability and incidence of E+A galaxies in the distant cluster Cl0939+472 (z=0.41).

Abstract

We present results of a study of E+A galaxies in the cluster Cl0939+472 (z=0.41). We have used a series of narrow-band filters (FWHM=~90-200A) covering the range from 3800 to 9200A, and broad band B, R, I filters to obtain low resolution spectra for all galaxies brighter than R=22.5mag in a 5x5' cluster field. Two of the narrow-band filters have been chosen to measure the 4000 A-break index, and 3 filters were centered on the H_beta_, H_gamma_ and H_delta_ lines which are indicators of recent star formation. Template spectra were fitted to the low-resolution spectral energy distributions in order to determine the redshift and to assign a corresponding morphological type for each galaxy in the sample. The E+A templates were derived from a model which assumes that a strong star formation episode took place in an early-type galaxy 1-2Gyr before its observational epoch (post starburst models). We detected 35 E+A galaxies in this part of the cluster, 21% of the total number of galaxies investigated. Their projected radial distribution shows that they are less concentrated than ellipticals. Moreover, we identified half of them on high resolution HST images (Dressler 1992, 1993). Their morphology indicates that this kind of activity is associated with disk-like systems or mergers. Two cluster members, an unusually blue elliptical galaxy and a merger, seem to be going through an early phase of the starburst evolution, in which the second burst is still ongoing.