A GALEX/Spitzer survey of the Cl 0016+16 supercluster at z = 0.55: acceleration of the onset of star formation in satellite groups

We present the results of a panoramic (15 Mpc scale) survey of the Cl 0016+16 (z = 0.55) supercluster using Spitzer Space Telescope Multiband Infrared Photometer (MIPS) 24-μm and Galaxy Evolution Explorer near-ultraviolet (2500 Å; NUV) imaging. The supercluster regions probed are characterized by several dense nodes connected by a pronounced intermediatedensity filamentary structure. We have studied the mid-infrared and NUV properties of potential cluster members within a z = 0.1 photometric redshift slice compared to an identical blank field selection. We have two main findings: (i) the star formation rates of individual star-forming galaxies throughout the cluster are not significantly different from identically selected field galaxies, and (ii) the cluster harbours pockets of ‘accelerated’ activity where galaxies have an enhanced probability of undergoing star formation. This observation could be explained in a simple model of ‘pre-processing’ of galaxies during cluster infall: galaxies in satellite groups have an increased chance of having star formation triggered via gravitational tidal interactions compared to their counterparts in the field, but there is no environmental mechanism boosting the individual star formation rates of galaxies. We estimate a lower limit for the total star formation rate of galaxies in the supercluster as ∼850 M yr−1 (field corrected). If this rate is maintained over the typical infall time of a few Gyr, then the infall population could contribute ∼1–2 × 1012 M of stellar mass to the structure.