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The Butcher - The Butcher-Oemler effect is not solely...

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The Butcher-Oemler effect is not solely redshift dependent, although it is nearly universal at z ~ 0.4 . There are a few very rich clusters of red galaxies at about z=0.4 (Cl 0024+1654, for example, Schneider, Dressler, & Gunn 1989 AJ 92, 523), and rich clusters like Abell 2125 at z=0.25 (an HST detail of part of this cluster is shown below) which are unusually rich in spiral members, so that the density history of a cluster may be more important than redshift per se . If so, we see the effect because what we consider typical clusters don't span an enormous range in the rate at which spirals are transformed. There is a pretty clear selection effect involved in tracing this phenomenon, related to the Scott effect, such that at higher redshift we sample systematically richer (and more X-ray luminous) clusters (Andreon & Ettori 1999 ApJ 516, 647, though I wouldn't agree with all their conclusions). Such cases as A2125-C153 can suggest a sequence of events associated with strong stripping episodes. Initially, the major effect may be compression of the ISM (and possibly global magnetic field as well), leading to an increase in the rate of star formation. Eventually, enough gas leaves the system to cause a drop in SFR. Detailed modelling shows that this loss will be very episodic, depending on the spin/cluster orbit geometry as well as internal instabilities (see animations from Bernd Vollmer). Eventually one expects something like an S0. A surprise has
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