THE ENVIRONMENTAL DEPENDENCE OF THE EVOLVING S0 FRACTION

We re-investigate the dramatic rise in the S0 fraction, f(S0), within clusters since z similar to 0.5. In particular, we focus on the role of the global galaxy environment on fS0 by compiling, either from our own observations or the literature, robust line-of-sight velocity dispersions, sigma's, for a sample of galaxy groups and clusters at 0.1 < z < 0.8 that have uniformly determined, published morphological fractions. We find that the trend of f(S0) with redshift is twice as strong for sigma < 750 km s(-1) groups/poor clusters than for higher-sigma, rich clusters. From this result, we infer that over this redshift range galaxy-galaxy interactions, which are more effective in lower-sigma environments, are more responsible for transforming spiral galaxies into S0's than galaxy-environment processes, which are more effective in higher-sigma environments. The rapid, recent growth of the S0 population in groups and poor clusters implies that large numbers of progenitors exist in low-sigma systems at modest redshifts (similar to 0.5), where morphologies and internal kinematics are within the measurement range of current technology.