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arXiv · 1110.3821

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

J. M. Lotz·C. Papovich·S. M. Faber·H. C. Ferguson·N. Grogin·Y. Guo·D. Kocevski·A. M. Koekemoer·K-S. Lee·D. McIntosh·I. Momcheva·G. Rudnick·A. Saintonge·K-V. Tran·A. van der Wel·C. Willmer
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Abstract

We present the recent merger history of massive galaxies in a spectroscopically-confirmed proto-cluster at z=1.62. Using HST WFC3 near-infrared imaging from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), we select cluster galaxies and z ~ 1.6 field galaxies with M_star >= 3 x 10^10 M_sun, and determine the frequency of double nuclei or close companions with projected separations less than 20 kpc co-moving and stellar mass ratios between 1:1 and roughly 10:1. We find that four out of five spectroscopically-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 either in close pair systems or have double nuclei. In contrast, only 4.5 +/- 2.6% of the field galaxies are in close pair/double nuclei systems. The implied merger rate per massive galaxy in the proto-cluster is 3-10 times higher than the merger rate of massive field galaxies at z ~ 1.6, depending upon the assumed mass ratios. Close pairs in the cluster have minor merger stellar mass ratios (M_primary:M_satellite ~ 6:1), while the field pairs are typically major mergers with stellar mass ratios between 1:1 and 4:1. At least half of the cluster mergers are dissipationless, as indicated by their red colors and low 24 micron fluxes. Two of the double-nucleated cluster members have X-ray detected AGN with L_x > 10^43 erg/s, and are strong candidates for dual or offset super-massive black holes. We conclude that the massive z = 1.62 proto-cluster galaxies are undergoing accelerated assembly relative to the field population, and discuss the implications for galaxy evolution in proto-cluster environments.

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J. M. Lotz, C. Papovich, S. M. Faber, H. C. Ferguson, N. Grogin, Y. Guo, D. Kocevski, A. M. Koekemoer, K-S. Lee, D. McIntosh, I. Momcheva, G. Rudnick, A. Saintonge, K-V. Tran, A. van der Wel, C. Willmer. 2011-10-17. Caught in the Act: The Assembly of Massive Cluster Galaxies at z=1.62. https://arxiv.org/abs/1110.3821

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