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Journal Articles The Astrophysical Journal Year : 2020

Spectroscopic Confirmation of a Coma Cluster Progenitor at z ∼ 2.2

Behnam Darvish
Nick Z. Scoville
  • Function : Author
Christopher Martin
  • Function : Author
David Sobral
  • Function : Author
Bahram Mobasher
  • Function : Author
Alessandro Rettura
  • Function : Author
Jorryt Matthee
Peter Capak
Nima Chartab
  • Function : Author
Shoubaneh Hemmati
  • Function : Author
Daniel Masters
Hooshang Nayyeri
Donal O'Sullivan
  • Function : Author
Ana Paulino-Afonso
  • Function : Author
Zahra Sattari
  • Function : Author
Abtin Shahidi
  • Function : Author
Mara Salvato
  • Function : Author
Brian C. Lemaux
  • Function : Author
Olga Cucciati
  • Function : Author


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 zmean = 2.23224 ± 0.00101 and σlos = 645 ± 69 km s-1 for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of Mvir ∼ (1-2) ×1014M for the structure. We evaluate a number density enhancement of δg ∼ 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, δg 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 Mdyn(z = 0) ∼ 9.2 × 1014M 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.

Dates and versions

insu-03667122 , version 1 (13-05-2022)



Behnam Darvish, Nick Z. Scoville, Christopher Martin, David Sobral, Bahram Mobasher, et al.. Spectroscopic Confirmation of a Coma Cluster Progenitor at z ∼ 2.2. The Astrophysical Journal, 2020, 892, ⟨10.3847/1538-4357/ab75c3⟩. ⟨insu-03667122⟩
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