The environmental dependence of the evolving S0 fraction

We re-investigate 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, σ′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 σ < 750 km s^-1 groups/poor clusters than for higher-σ, rich clusters. From this result, we infer that over this redshift range galaxy-galaxy interactions, which are more effective in lower-σ environments, are more responsible for transforming spiral galaxies into S0's than galaxy-environment processes, which are more effective in higher-σ environments. The rapid, recent growth of the S0 population in groups and poor clusters implies that large numbers of progenitors exist in low-σ systems at modest redshifts (0.5), where morphologies and internal kinematics are within the measurement range of current technology.