Quantitative morphology and color gradients of E+A galaxies in distant galaxy clusters

Three basic scenarios have been invocated to explain the sudden rise and decline of star formation in distant clusters that lead to the transformation of " active " galaxies into the elliptical− and S0−types which today dominate rich clusters, i.e. the so called Butcher-Oemler effect, (Butcher & Oemler,1978). These are: galaxy interactions or mergers with nearly equal mass neighbors (Lavery & Henry, 1988), gas-rich field galaxies running for the first time into the hot intercluster gas which ignites a brief but energetic episode of star formation (Bothun & Dressler, 1986), and high speed close encounters of gas-rich galaxies resulting in non-disruptive interactions or " galaxy harassment " , (Moore, 1996). Although neither of the proposed mechanisms for triggering star formation has a definitive answer in its favour, the high resolution of the HST images of distant clusters has recently provided a unique tool to face this question. Indeed, it allows us to study the spatial distribution of the starburst and thus to distinguish among the physical mechanisms at its origin. In the case of an infalling or harassed spiral the enhanced star formation would likely be a galaxy-wide phenomenon, or confined to the disk. If, on the other hand, the original galaxy was an elliptical that accreted gas from a dwarf galaxy, the burst signatures should be detectable as a bluer color of the nuclear region. Finally, in interacting or merging disk galaxies both behaviours have been observed: galaxies with starburst concentrated to the very center (Scoville et al.,1991) and others that show enhanced star formation on a galaxy-wide scale (Standford, 1991). We focuse on one class of " active " galaxies, the post-starburst or E+A galaxies , the latter being only a description of the spectra which appear to have an A-star component added to an old elliptical like component. These spectral features are interpreted as evidence of a recent (< 1.5 Gyr) burst of star formation. Ground based narrow-band photometry of 4 intermediate redshift clusters at z=0.4-0.5 has provided us with a sample of 73 E+A galaxies being secure cluster members (Belloni et al.,1995; Belloni & Röser, 1996). This is the largest sample of such galaxies found to date in distant clusters. Cluster membership and spectral type have been obtained by fitting the observed low-resolution spectral energy distributions with template spectra built up with Bruzual & Charlot (1997) population synthesis models. They represent the temporal evolution of a …