Spectroscopic Confirmation of a Coma Cluster Progenitor at z ~ 2.2
  We report the spectroscopic confirmation of a new protocluster in the COSMOS
field at $z$ $\sim$ 2.2, COSMOS Cluster 2.2 (CC2.2), originally identified as
an overdensity of narrowband selected H$\alpha$ emitting candidates. With only
two masks of Keck/MOSFIRE near-IR spectroscopy in both $H$ ($\sim$ 1.47-1.81
$\mu$m) and $K$ ($\sim$ 1.92-2.40 $\mu$m) bands ($\sim$ 1.5 hour 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 $\pm$ 0.00101 and
$\sigma_{los}$=645 $\pm$ 69 km s$^{-1}$ for this protocluster, respectively.
Assuming virialization and spherical symmetry for the system, we estimate a
total mass of $M_{vir}$ $\sim$ $(1-2) \times$10$^{14}$ $M_{\odot}$ for the
structure. We evaluate a number density enhancement of $\delta_{g}$ $\sim$ 7
for this system and we argue that the structure is likely not fully virialized
at $z$ $\sim$ 2.2. However, in a spherical collapse model, $\delta_{g}$ is
expected to grow to a linear matter enhancement of $\sim$ 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) $\sim$
9.2$\times$10$^{14}$ $M_{\odot}$ 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.
