Spectroscopic Confirmation of a Coma Cluster Progenitor at z ∼ 2.2
Behnam Darvish, Nick Z. Scoville, Christopher Martin, David Sobral, Bahram Mobasher, Alessandro Rettura, Jorryt Matthee, Peter Capak, Nima Chartab, Shoubaneh Hemmati, Daniel Masters, Hooshang Nayyeri, Donal O’Sullivan, Ana Paulino-Afonso, Zahra Sattari, Abtin Shahidi, Mara Salvato, Brian C. Lemaux, Olivier Le Fèvre, Olga Cucciati
Abstract
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 H α emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both H (∼1.47–1.81 μ m) and K (∼1.92–2.40 μ m) 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) ×10 14 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 × 10 14 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.