Testing the Hypothesis of the Morphological Transformation from Field   Spiral to Cluster S0
  Hubble Space Telescope observations of distant clusters have suggested a steep increase in the proportion of S0 galaxies between clusters at high redshifts and similar systems the present-day. It has been proposed that this increase results from the transformation of the morphologies of accreted field galaxies from spirals to S0s. We have simulated the evolution of the morphological mix in clusters based on a simple phenomenological model where the clusters accrete a mix of galaxies from the surrounding field, the spiral galaxies are transformed to S0s (through an unspecified process) and are added to the existing cluster population. We find that in order to reproduce the apparently rapid increase in the ratio of S0 galaxies to ellipticals in the clusters, our model requires that: (1) galaxy accretion rate has to be high (typically, more than half of the present-day cluster population must have been accreted since z~0.5), and (2) most of the accreted spirals, with morphological types as late as Scdm, must have transformed to S0's. Although the latter requirement may be difficult to meet, it is possible that such bulge-weak spirals have already been `pre-processed' into the bulge-strong galaxies prior to entering the cluster core and are eventually transformed into S0's in the cluster environment. On the basis of the evolution of the general morphological mix in clusters our model suggests that the process responsible for the morphological transformation takes a relatively long time (1-3Gyr) after the galaxy has entered the cluster environment. 
Tadayuki Kodama (1,2), Ian Smail (1) ((1) Univ of Durham, (2) Univ of   Tokyo)