Galaxy clusters are the most massive gravitationally bound structures in the Universe and they play a key role in our understanding of structure formation and galaxy evolution. The particular conditions of these high density environments can have a significant effect on the evolution of galaxies. Previous studies have reported systematic differences in SFR, age and mass between field galaxies and galaxies in large clusters at z<2.5. However, we know little about how overdense environments affect the evolution of galaxies at z > 3.

We propose to use 15 h of NIRSpec and NIRCam parallel observations to measure optical nebular line emission properties for a well-defined sample of proto-cluster galaxies at z=5.2. Our target sample contains 11 spectroscopically confirmed members of the proto-cluster and 15 new candidates with accurate photometric redshifts, as well as 66 other field galaxies at similar redshifts, in order to: 1) use emission line diagnostics and stellar population modeling to directly compare properties of galaxies in high- and lower-density environments for the first time at this early cosmic epoch; 2) substantially increase the number of confirmed proto-cluster members, allowing a detailed characterization of the dynamical state of the overdensity, the identification of internal sub-structures through the analysis of the velocity dispersion, and application of cluster evolution formalism to trace the expected evolution of this large proto-cluster with redshift, and its eventual end as a Coma-like cluster at z=0; and 3) compare the Ly-alpha escape fraction for z~5 galaxies in and out of the porto-cluster environment.