The cutting off of gas supply in galaxies, whether it is exhausted internally or removed forcibly, can have dramatic consequences and are thought to build up clusters dominated by an early-type galaxy population, as observed in the local Universe. In recent years it has become more and more obvious that this transformation starts in moderate overdensities and filaments surrounding and feeding galaxy clusters. Keeping in mind that the environment of a galaxy cluster is the result of continuous hierarchical assembly boasting a wide range of substructures of locally dense environments, we therefore need to ask: Are the observed galaxy (scaling-) relations due to the timing and physics of cosmological structure formation or due to baryonic physics?

To answer this question we need to look beyond the cluster core.
We therefore investigated members (down to 10^8.5 M_sol) of a massive galaxy cluster at z=0.44 out to 3 virial radii. Our observations probe the nearby infall region, central to the topical question of “pre-processing”. The smoking gun of this “pre-processing” are transitional objects, like passive disk galaxies, whose stellar populations and morphologies indicate a recent change in star formation and/or dynamical history. We find a large number of this virialized population at distinct locations. In our exploration of the galaxy stellar mass-size relation (Kuchner+17), we were able to show that the manifestation of the size distribution of the cluster galaxies is due to an outer disk-fading and possible bulge growth that accompanies the varying fraction of star-forming and quiescent galaxies.

Continuing this quest, the Wiliam Herschel Telescope’s new multi-object survey spectrograph WEAVE will run a dedicated survey that focuses on this question of “pre-processing”. WEAVE will map 16 nearby galaxy clusters and their filamentary structure of infalling galaxies out to 5 viral radii. This will allow us to characterize the onset and continuation of galaxy transformation during their infall process toward the cluster center.