Dynamics of Star-forming Galaxies in a Massive Structure at \lowercase{$z\sim$} 2.2: Evidence for Galaxy Harassment in high-$z$ Environments

Abstract

We spectroscopically confirm a new protocluster in the COSMOS field at $z$=2.24430 with Keck/MOSFIRE, dubbed CC2.2B, which is in the immediate vicinity of CC2.2A protocluster, originally presented in \cite{Darvish20}. CC2.2B and CC2.2A centroids are separated by $\sim$5.5 Mpc(angular) and $\sim$16 comoving Mpc(radial). CC2.2B and CC2.2A have similar properties, with CC2.2B having a line-of-sight velocity dispersion and estimated total mass of $\ddot{I}{ƒ}_{los}$=693$\pm$65 km s${}^{-1}$ and $M_{total}$=($\sim$2-3)$\times$10${}^{14}$ $M_{\odot }$, respectively. These two similar overdensities are likely still in the merging process and will likely collapse into a more massive structure at lower redshifts. We combine CC2.2A and CC2.2B data to investigate the role of high-$z$ protocluster environments on the dynamics of star-forming (SF) galaxies compared to a similarly selected field sample. We find that on average, protocluster SF galaxies have $\sim$0.1 dex (at $\sim$1.8$\ddot{I}ƒ$ significance) lower gas velocity dispersions, $\sim$0.2 dex (at $\sim$2.2$\ddot{I}ƒ$ significance) lower dynamical masses, and $\sim$0.2 dex lower dynamical-to-stellar mass ratio than the field SF galaxies. We argue that galaxy harassment and galaxy-galaxy interactions can potentially explain these differences. We also find a factor of $\sim$2-3 lower scatter around the mean $\ddot{I}ƒ$-$M_{\ast }$, $M_{dyn}$-$M_{\ast }$, and $M_{dyn}$/$M_{\ast }$ vs. $M_{\ast }$ relations for protocluster SF galaxies than the field. This could be due to a more uniform formation for protocluster galaxies than their field counterparts. Our results have potential implications for the physics of preprocessing in early environments.