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 narrow-band 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 narrow-band 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 .