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D. H. McIntosh (UMass), H.-W. Rix (MPIA), N. Caldwell, (Smithsonian)
A large body of observational evidence over the past 20 years, plus CDM cosmological simulations, have led astronomers to hypothesize that much of the current cluster galaxy membership resulted from the fairly rapid (1-2 Gyrs) transformation of normal, infalling spirals into red, cluster early-types. A consequence of this transformation should be a population of galaxies intermediate between field spiral and red cluster member. Therefore, we have conducted a wide-field, photometric search for intermediate aged galaxies within three local (z<0.06) Abell clusters: A85, A496 and A754. Using color-magnitude based criteria to estimate relative age, we found significant (10%) populations of intermediate blue galaxies (IBG's) in two clusters (A85 and A754). In addition, there exists similar numbers of very blue galaxies (VBG's) in all three clusters. The observed photometric and structural properties of these galaxies are consistent with a sequence of initial (VBG's) to mid (IBG's) transformation via galaxy harassment and/or ram-pressure stripping. Thus, the existence of these galaxies provides clear observational evidence for the present-day whereabouts of the blue galaxies once prevalent in rich clusters (the Butcher-Oemler effect) and for the environmental based evolution of the cluster galaxy membership.
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