Galaxy evolution near groups and clusters: Ejected satellites and the spatial extent of environmental quenching

Galaxies that are several virial radii beyond groups/clusters show preferentially quiescent star formation rates (SFR). Using a galaxy group/cluster catalogue from the Sloan Digital Sky Survey, together with a cosmologicalN-body simulation, we examine the origin of this environmental quenching beyond the virial radius. Accounting for the clustering of groups/clusters, we show that central galaxies show enhanced SFR quenching out to 2.5 virial radii beyond groups/clusters, and we demonstrate that this extended environmental enhancement can be explained simply by 'ejected' satellite galaxies that orbit beyond their host halo's virial radius. We show that ejected satellites typically orbit for several Gyr beyond the virial radius before falling back in, and thus they compose up to 40 per cent of all central galaxies near groups/clusters. We show that a model in which ejected satellites experience the same SFR quenching as satellites within a host halo can explain essentially all environmental dependence of galaxy quenching. Furthermore, ejected satellites (continue to) lose significant halo mass, an effect that is potentially observable via gravitational lensing. The SFRs/colours and stellar-to-halo masses of ejected satellites highlight the importance of environmental history and present challenges to models of galaxy occupation that ignore such history. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society