Spectroscopic Observations of Galaxies in A370

Abstract

The observation of distant clusters of galaxies is the best way to understand the evolution of galaxies in clusters. Since about 5 years, the photometric and spectroscopic studies of these systems have largely confirmed that the blue galaxy excess first mentionned by Butcher and Oemler [1] is common in distant rich clusters of galaxies. However, the origin of this effect is always unclear. Moreover, the first spectroscopic observations of Dressier and 6unn [2,3] have shown that: (1) this effect present a large discrepancy from cluster to cluster either quantitatively or in their spectral content (AGN or normal spirals). (2) In addition to the blue galaxies, distant clusters have a large extend of spectral types as the Post-Star-Burst galaxies (E+A) or SO with UV-excess [4]. The understanding of the evolution of galaxies in clusters remains a challenge: are these blue galaxies result from galaxy/gas interactions or galaxy/galaxy interactions due to close encounters ? Are they young normal galaxies falling inward the cluster center or old cluster members which have undergone star-burst formation? Although, spectral indications inform us on possible scenarios, not enough data are presently available to firmly conclude on the evolutionary processes responsible of this spectral content in clusters of galaxies. A larger sample of clusters with very good data is still needed.