The transformation of galaxies within the large-scale structure around a z=0.41 cluster

We present deep, panoramic multicolor imaging of the distant rich cluster A 851 (Cl 0939+4713, z = 0.41) using Suprime-Cam on the Subaru Telescope. The se images cover a 27' field of view, similar to 11 h(50)(-1) Mpc at z = 0.4 1, and by exploiting photometric redshifts estimated from our BVRI imaging we can isolate galaxies in a narrow redshift slice at the cluster redshift. Using a sample of similar to 2700 probable cluster members brighter than 0 .02L(v)*, we trace the network of filaments and subclumps around the cluste r core. The depth of our observations, combined with the identification of filamentary structure, gives us an unprecedented opportunity to test the in fluence of the environment on the properties of low-luminosity galaxies. We find an abrupt change in the colors of less than or similar to0.1L(v)* gal axies at a local density of 100 galaxies Mpc(-2), with the population in lo wer density regions being predominantly blue while those in higher density regions are red. The transition in the color-local density behavior occurs at densities corresponding to subclumps within the filaments surrounding th e cluster. Identifying the sites where the transition occurs brings us much closer to understanding the mechanisms that are responsible for establishi ng the present-day relationship between environment and galaxy characterist ics.