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The low redshift Universe (z<~0.5) is not a dull place. Processes leading tothe suppression of star formation and morphological transformation areprevalent: this is particularly evident in the dramatic upturn in the fractionof S0-type galaxies in clusters. However, until now, the process andenvironment of formation has remained unidentified. We present a HST-basedmorphological analysis of galaxies in the redshift-space selected group andfield environments at z~0.4. Groups contain a much higher fraction of S0s atfixed luminosity than the lower density field, with >99.999% confidence. Indeedthe S0 fraction in groups is at least as high as in z~0.4 clusters and X-rayselected groups, which have more luminous Intra Group Medium (IGM). An 97%confident excess of S0s at >=0.3Mpc from the group centre at fixed luminosity,tells us that formation is not restricted to, and possibly even avoids, thegroup cores. Interactions with a bright X-ray emitting IGM cannot be importantfor the formation of the majority of S0s in the Universe. In contrast to S0s,the fraction of elliptical galaxies in groups at fixed luminosity is similar tothe field, whilst the brightest ellipticals are strongly enhanced towards thegroup centres (>99.999% confidence within 0.3Mpc). We conclude that the groupand sub-group environments must be dominant for the formation of S0 galaxies,and that minor mergers, galaxy harassment and tidal interactions are the mostlikely responsible mechanisms. This has implications not only for the inferredpre-processing of cluster galaxies, but also for the global morphological andstar formation budget of galaxies: as hierarchical clustering progresses, moregalaxies will be subject to these transformations as they enter the groupenvironment.

Morphological Composition of z~0.4 groups: The site of S0 formation