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We present the recent merger history of massive galaxies in aspectroscopically-confirmed proto-cluster at z=1.62. Using HST WFC3near-infrared imaging from the Cosmic Assembly Near-infrared Deep ExtragalacticLegacy Survey (CANDELS), we select cluster galaxies and z ~ 1.6 field galaxieswith M_star >= 3 x 10^10 M_sun, and determine the frequency of double nuclei orclose companions with projected separations less than 20 kpc co-moving andstellar mass ratios between 1:1 and roughly 10:1. We find that four out of fivespectroscopically-confirmed massive proto-cluster galaxies have double nuclei,and 42 +13/-25 % of all M_star >= 3 x 10^10 M_sun cluster candidates are eitherin close pair systems or have double nuclei. In contrast, only 4.5 +/- 2.6% ofthe field galaxies are in close pair/double nuclei systems. The implied mergerrate per massive galaxy in the proto-cluster is 3-10 times higher than themerger rate of massive field galaxies at z ~ 1.6, depending upon the assumedmass ratios. Close pairs in the cluster have minor merger stellar mass ratios(M_primary:M_satellite ~ 6:1), while the field pairs are typically majormergers with stellar mass ratios between 1:1 and 4:1. At least half of thecluster mergers are dissipationless, as indicated by their red colors and low24 micron fluxes. Two of the double-nucleated cluster members have X-raydetected AGN with L_x > 10^43 erg/s, and are strong candidates for dual oroffset super-massive black holes. We conclude that the massive z = 1.62proto-cluster galaxies are undergoing accelerated assembly relative to thefield population, and discuss the implications for galaxy evolution inproto-cluster environments.

Caught in the Act: The Assembly of Massive Cluster Galaxies at z=1.62