Spectroscopic Confirmation of a Coma Cluster Progenitor at z ∼ 2.2

• Published in: The Astrophysical journal 2020-03, Vol.892 (1), p.8
• Main Authors: Darvish, Behnam, Scoville, Nick Z., Martin, Christopher, Sobral, David, Mobasher, Bahram, Rettura, Alessandro, Matthee, Jorryt, Capak, Peter, Chartab, Nima, Hemmati, Shoubaneh, Masters, Daniel, Nayyeri, Hooshang, O'Sullivan, Donal, Paulino-Afonso, Ana, Sattari, Zahra, Shahidi, Abtin, Salvato, Mara, Lemaux, Brian C., Fèvre, Olivier Le, Cucciati, Olga
• Format: Article
• Language: English
• Subjects: Astrophysics; Cosmos; Emission lines; Emissions; Galactic clusters; Galaxies; Galaxy clusters; Galaxy environments; Galaxy evolution; High-redshift galaxies; High-redshift galaxy clusters; Infrared spectroscopy; Large scale structure of the universe; Line of sight; Masks; Narrowband; Near infrared radiation; Polls & surveys; Red shift; Sciences of the Universe; Spectroscopy; Star clusters; Star formation
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab75c3

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 emitting candidates. With only two masks of Keck/MOSFIRE near-IR spectroscopy in both H (∼1.47-1.81 m) and K (∼1.92-2.40 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 zmean = 2.23224 0.00101 and los = 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.