Caught in the act: The assembly of massive cluster galaxies at z = 1.62

Jennifer M. Lotz, Casey Papovich, S. M. Faber, Henry C. Ferguson, Norman Grogin, Yicheng Guo, Dale Kocevski, Anton M. Koekemoer, Kyoung Soo Lee, Daniel McIntosh, Ivelina Momcheva, Gregory Rudnick, Amelie Saintonge, Kim Vy Tran, Arjen Van Der Wel, Christopher Willmer

Research output: Contribution to journalArticlepeer-review

59 Scopus citations


We present the recent merger history of massive galaxies in a spectroscopically confirmed proto-cluster at z = 1.62. Using Hubble Space Telescope WFC3 near-infrared imaging from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we select cluster and z ∼ 1.6 field galaxies with M star ≥ 3 × 1010 M , to determine the frequency of double nuclei or close companions within projected separations less than 20 kpc co-moving. We find that four out of five spectroscopically confirmed massive proto-cluster galaxies have double nuclei, and 57 % of all M star ≥ 3 × 1010 M cluster candidates are observed in either close pair systems or have double nuclei. In contrast, only 11% ± 3% of the field galaxies are observed in close pair/double nuclei systems. After correcting for the contribution from random projections, 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. Close pairs in the cluster have minor merger stellar mass ratios (M primary: M satellite ≥ 4), while the field pairs consist of both major and minor mergers. At least half of the cluster mergers are gas-poor, as indicated by their red colors and low 24 μm fluxes. Two of the double-nucleated cluster members have X-ray detected active galactic nuclei with Lx > 1043 erg s-1, 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 via minor mergers, and discuss the implications for galaxy evolution in proto-cluster environments.

Original languageEnglish (US)
Article number154
JournalAstrophysical Journal
Issue number2
StatePublished - Aug 20 2013


  • galaxies: clusters: general
  • galaxies: high-redshift
  • galaxies: interactions

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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