The Environmental Dependence of the Evolving S0 Fraction
  We reinvestigate the dramatic rise in the S0 fraction, f_S0, within clusters
since z ~ 0.5. In particular, we focus on the role of the global galaxy
environment on f_S0 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 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 (~ 0.5), where morphologies and internal kinematics are
within the measurement range of current technology.
