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 sigma_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) x 10^14 M_sun 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 x 10^14 M_sun 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.