Is there a deficit of S0s at intermediate redshift?

Abstract

Two contradictory results on the evolution of SO galaxies now exist in the recent literature; either S0s are old ($z_{formation}>2$) and are evoloving passively or most of them formed at $z<0.5$, as implied by the deficit of S0s in intermediate redshift ($z\sim0.5$) clusters. The resolution of this controversy may be that the apparent deficit of S0s has been derived from a quantity -- the E to S0 ratio -- which is prone to morphological classification errors. Once all sources of error are taken into account, the E to S0 ratios of clusters at different redshifts are fully compatible, and no additional creation of S0s at $z<0.5$ is required by the data. Furthermore, there is no deficit at all of S0s in the intermediate redshift cluster for which we have morphological types of very high quality, and thus derive an E to S0 ratio with a small error.