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Published March 20, 2020 | Published + Submitted
Journal Article Open

Spectroscopic Confirmation of a Coma Cluster Progenitor at z ∼ 2.2

Abstract

We report the spectroscopic confirmation of a new protocluster in the COSMOS field at z ~ 2.2, COSMOS Cluster 2.2 (CC2.2), originally identified as an overdensity of narrowband selected Hα emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both H (~1.47–1.81 μm) and K (~1.92–2.40 μm) bands (~1.5 hr each), we confirm 35 unique protocluster members with at least two emission lines detected with S/N > 3. Combined with 12 extra members from the zCOSMOS-deep spectroscopic survey (47 in total), we estimate a mean redshift and a line-of-sight velocity dispersion of z_(mean) = 2.23224 ± 0.00101 and σ_(los) = 645 ± 69 km s−1 for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of M_(vir) ~ (1–2) ×10¹⁴ M⊙ for the structure. We evaluate a number density enhancement of δ g ~ 7 for this system and we argue that the structure is likely not fully virialized at z ~ 2.2. However, in a spherical collapse model, δ g is expected to grow to a linear matter enhancement of ~1.9 by z = 0, exceeding the collapse threshold of 1.69, and leading to a fully collapsed and virialized Coma-type structure with a total mass of M_(dyn)(z = 0) ~ 9.2 × 10¹⁴ M⊙ by now. This observationally efficient confirmation suggests that large narrowband emission-line galaxy surveys, when combined with ancillary photometric data, can be used to effectively trace the large-scale structure and protoclusters at a time when they are mostly dominated by star-forming galaxies.

Additional Information

© 2020 The American Astronomical Society. Received 2019 July 27; revised 2020 January 17; accepted 2020 February 10; published 2020 March 19. We are thankful to the anonymous referee for useful comments and suggestions that improved the quality of this paper. B.D. acknowledges financial support from NASA through the Astrophysics Data Analysis Program (ADAP), grant number NNX12AE20G, and the National Science Foundation, grant number 1716907. B.D. is thankful to Andreas Faisst, Laura Danly, and Matthew Burlando for their companionship during the observing run. B.D. is grateful to the COSMOS team for their useful comments during the team meeting in New York City 2019 May 14–17. A.R. research was made possible by Friends of W. M. Keck Observatory who philanthropically support the Keck Science Collaborative (KSC) fund. The observations presented herein were obtained at the W. M. Keck Observatory (program C236, PI Scoville), which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors would like to recognize and acknowledge the very prominent cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are fortunate to have the opportunity to perform observations from this mountain.

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Published - Darvish_2020_ApJ_892_8.pdf

Submitted - 2002.06207.pdf

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August 22, 2023
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October 19, 2023