2017ApJ...834..205M

We investigate star formation in DLSCL J0916.2+2953, a dissociative merger of two clusters at z = 0.53 that has progressed 1.1_–0.4^+1.3 Gyr since the first pass-through. We attempt to reveal the effects a collision may have had on the evolution of the cluster galaxies by tracing their star formation history. We probe current and recent activity to identify a possible star formation event at the time of the merger, using EW(Hδ), EW([O II]), and D_n(4000) measured from the composite spectra of 64 cluster and 153 coeval field galaxies. We supplement Keck DEep Imaging Multi-Object Spectrograph spectra with DLS and Hubble Space Telescope imaging, to determine the color, stellar mass, and morphology of each galaxy. We also conduct a comprehensive study of the populations in this complex structure. Spectral results indicate the average cluster and cluster red sequence galaxies experienced no enhanced star formation relative to the surrounding field during the merger, ruling out a predominantly merger-quenched population. We find that the average blue galaxy in the North cluster is currently active, and that the South cluster is currently post-starburst, having undergone a recent star formation event. Although the North activity could be latent or long-term merger effects, a young blue stellar population and irregular geometry suggest the cluster was still forming prior the collision. Even though the South activity coincides with the time of the merger, the blue early-type population could be a result of secular cluster processes. The evidence suggests that the dearth or surfeit of activity is indiscernible from normal cluster galaxy evolution.