LoCuSS : luminous infrared galaxies in the merging cluster Abell 1758 at z = 0 . 28

We present the first galaxy evolution results from the Local Cluster Substructure Survey (LoCuSS), a multiwavelength survey of 100 X-ray selected galaxy clusters at 0.15 ≤ z ≤ 0.3. LoCuSS combines far-ultraviolet (UV) through far-infrared (IR) observations of cluster galaxies with gravitational lensing analysis and X-ray data to investigate the interplay between the hierarchical assembly of clusters and the evolution of cluster galaxies. Here we present new panoramic Spitzer/Multiband Imaging Photometer 24-μm observations of the merging cluster Abell 1758 at z = 0.279 spanning 6.5 × 6.5 Mpc2 and reaching a 90 per cent completeness limit of S24μm = 400μJy. We estimate a global cluster star formation rate of SFR24μm = 910 ± 320 M yr−1 within R < 3 Mpc of the cluster centre, originating from 42 galaxies with L8−1000μm > 5 × 1010 L . The obscured activity in A1758 is therefore comparable with that in Cl 0024+1654, the most active cluster previously studied at 24μm. The obscured galaxies faithfully trace the cluster potential as revealed by the weak-lensing mass map of the cluster, including numerous mass peaks at R ∼ 2–3 Mpc that are likely associated with infalling galaxy groups and filamentary structures. However, the core (R 500 kpc) of A1758N is ∼two times more active in the IR than that of A1758S, likely reflecting differences in the recent dynamical history of the two clusters. The 24-μm results from A1758 therefore suggest that dust-obscured cluster galaxies are common in merging clusters and suggest that obscured activity in clusters is triggered by both the details of cluster–cluster mergers and processes that operate at larger radii including those within in-falling groups. Our ongoing far-UV through far-IR observations of a large sample of clusters should allow us to disentangle the different physical processes responsible for triggering obscured star formation in clusters.