Witnessing pre-processing at intermediate redshift: observational evidence of infalling groups onto a merging galaxy cluster
The fraction of quenched galaxies increases with local density, and part of this trend is thought to arise in groups before cluster accretion (pre-processing). In merging clusters, this is difficult to separate from star formation triggered or suppressed by the cluster merger itself. We investigate whether five high-velocity galaxies in the merging cluster SPT-CL~J0307-6225 ($z=0.579$), distributed in two candidate infalling groups, show signs of group-scale environmental processing prior to accretion. We combine MUSE integral-field spectroscopy and HST imaging to derive [\ion{O}{ii}] kinematic maps, spectral classes, and star-formation rates (SFR), to understand the properties of the galaxies. We further use the Magneticum simulations and the phase-space information to determine if these galaxies are part of two groups, and if they will be accreted onto the merging cluster. Group~1 is a morphologically disturbed interacting pair bound to the cluster, with at least one galaxy classified as dusty starburst. Group~2 is a compact triplet at $\sim$4$σ_v$ from the cluster, in which two galaxies share a common [\ion{O}{ii}] envelope and the dominant member, an e(b) starburst, shows an elevated ionised-gas velocity dispersion. The SFR of both starburst galaxies lie within the scatter of the field star-forming main sequence, as expected for modest, interaction-triggered bursts. Close pairs at the phase-space position of group~2 occur for $\sim$1\% of comparable-mass cluster galaxies in Magneticum, so it is not unlikely for the group to be bound. Both groups show spatially resolved signatures of galaxy--galaxy interactions alongside ongoing star formation, supporting a pre-processing scenario in the environment of a merging cluster. Such group-scale interactions may contribute to the enhanced star formation reported in merging clusters.