GMOS SV program 61

The aim is to study the environmental dependence of galaxy evolution by studying the Butcher-Oemler effect (eg Butcher & Oemler, 1978, Dressler & Gunn, 1982, Couch & Sharples, 1987) in environments intermediate between rich clusters and the field. Hierarchical models for structure formation suggest that rich clusters are made of assemblages of groups (e.g. Kauffman et al) and that groups observed at intermediate redshift hold important clues to the build up of the rich clusters and the evolution of galaxies within locally dense environments. Photometric studies by Allington-Smith et al (1993) give statistical properties of the environments of 100 radio galaxies with z < 0.5, of which one-third are at intermediate redshift (z~0.4). Radio-selection provides an optically unbiassed method of uniformly selecting suitable environments at these redshifts. HST images provide morphological information. Although several mechanism have been suggested for the origin of these episodes of intense star formation, studies based on HST imaging (eg., Dressler et al 1997, Couch et al 1998) have been unable to single out a particular cause. The most likely mechanisms are ram pressure stripping, galaxy harassment and galaxy mergers. We cannot distinguish between these models without taking a broader perspective by studying the putative building blocks of rich clusters - poor groups - because these are favoured sites for galaxy-galaxy interactions and represent the most common type of galaxy environment since the field is actually composed of such groups. To do this we need spectroscopy to identify interactions from their spectroscopic signature (correlated with morphological information from HST) and to study group dynamics.

The 5C6.142 group is the best group to study in our sample because it is one of the richer (statistically) and most likely to yield plentiful confirmed members. Good HST images are already available but we urgently need more information on group membership to make sense of the HST data. The problem with this group is the relatively high redshift that has resulted in poor SNR in our previous attempts at spectroscopy on 4m telescopes. GMOS could really make a difference here! Without the light-grasp of GEMINI/GMOS, our study will be biassed towards the brighter and nearer groups.

GMOS is well-matched to the size of the group and its high spectral resolution and large spectrum length (up to 6144 pixels) will enhance the detectability of the diagnostic Balmer lines and improve radial velocities for studies of the group dynamics. Thus this SV observation would provide a limiting test on GMOS's ability to provide good spectral diagnostics on intermediate distance objects in multiobject mode. Targets will be selected on the basis of existing multi-band photometry using positions from pre-imaging with GMOS.