The Role of Groups in Galaxy Evolution: compelling evidence of pre-processing out to the turnaround radius of clusters

ABSTRACT We present clear and direct evidence of the pre-processing effect of group galaxies falling into clusters in the local Universe (z ≲ 0.1). We start with a sample of 238 clusters, from which we select 153 with N200 ≥ 20. We considered 1641 groups within the turnaround radius (∼5 × R200) of these 153 clusters. There are 6654 individual cluster galaxies and 4133 group galaxies within this radius. We considered two control samples of galaxies, in isolated groups and in the field. The former comprises 2601 galaxies within 1606 isolated groups, and the latter has 4273 field objects. The fraction of star-forming galaxies in infalling groups has a distinct clustercentric behaviour in comparison to the remaining cluster galaxies. Even at 5 × R200 the group galaxies already show a reduced fraction of star-forming objects. At this radius, the results for the individual cluster galaxies are actually compatible with the field. That is strong evidence that the group environment is effective to quench the star formation prior to the cluster arrival. The group star-forming fraction remains roughly constant inwards, decreasing significantly only within the cluster R200 radius. We have also found that the pre-processing effect depends on the group mass (indicated by the number of members). The effect is larger for more massive groups. However, it is significant even for pairs and triplets. Finally, we find evidence that the time-scale required for morphological transformation is larger than the one for quenching.