Spectroscopic confirmation of a Coma cluster progenitor at z ~ 2.2
Darvish, B., Scoville, N., Martin, C., Sobral, D., Mobasher, B., Rettura, A., Matthee, J., Capak, P., Chartab, N., Hemmati, S., Masters, D., Nayyeri, H., O'Sullivan, D., Paulino-Afonso, A., Sattari, Z., Shahidi, A., Salvato, M., Lemaux, B., Le Fèvre, O., Cucciati, O.
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 <sub>mean</sub> = 2.23224 ± 0.00101 and σ <sub>los</sub> = 645 ± 69 km s<sup>−1</sup> for this protocluster, respectively. Assuming virialization and spherical symmetry for the system, we estimate a total mass of M <sub>vir</sub> ~ (1–2) ×10<sup>14</sup> M <sub>⊙</sub> for the structure. We evaluate a number density enhancement of δ <sub>g</sub> ~ 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 <sub>g</sub> 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 <sub>dyn</sub>(z = 0) ~ 9.2 × 10<sup>14</sup> M <sub>⊙</sub> 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.