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We present data for 16 galaxies in the overdensity JKCS 041 at $z \simeq1.80$ as part of the K-band Multi-Object Spectrograph (KMOS) Cluster Survey(KCS). With 20-hour integrations, we have obtained deep absorption-line spectrafrom which we derived velocity dispersions for seven quiescent galaxies. Wecombined photometric parameters derived from Hubble Space Telescope images withthe dispersions to construct a fundamental plane (FP) for quiescent galaxies inJKCS 041. From the zero-point evolution of the FP, we derived a formationredshift for the galaxies of $z_{form} = 3.0\pm0.3$, corresponding to a meanage of $1.4\pm0.2$ Gyrs. We tested the effect of structural and velocitydispersion evolution on our FP zero point and found a negligible contributionwhen using dynamical mass-normalized parameters ($\sim 3\%$), but a significantcontribution from stellar-mass-normalized parameters ($\sim 42 \%$). From therelative velocities of the galaxies, we probed the three-dimensional structureof these 16 confirmed members of JKCS 041, and found that a group of galaxiesin the south west of the overdensity had systematically higher velocities. Wederived ages for the galaxies in the different groups from the FP. We found theeast-extending group had typically older galaxies ($2.1\substack{+0.3\\-0.2}$Gyrs), than those in the south-west group ($0.3\pm0.2$ Gyrs). Although based onsmall numbers, the overdensity dynamics, morphology, and age results couldindicate that JKCS 041 is in formation and may comprise two merging groups ofgalaxies. The result could link large-scale structure to ages of galaxies forthe first time at this redshift.

The KMOS Cluster Survey (KCS) III: fundamental plane of cluster galaxies at $z \simeq 1.80$ in JKCS 041