Mass--size evolution and the emerging passive--density relation revealed by JWST/NIRCam in the Spiderweb protocluster

We investigate how the environment affects galaxy structure in the Spiderweb protocluster at $z=2.16$ using JWST/NIRCam F115W, F182M, and F410M imaging (rest-frame $\sim 3500$Å to $1.4\mu$m). We perform homogeneous multi-wavelength parametric modelling of the single-Sérsic and bulge–disc decomposition for the Spiderweb member sample of 103 galaxies within the JWST field of view (up to $\sim 2\times R_{200}$). Star-forming galaxies follow a mass–size relation broadly consistent with the field, with a mildly steeper trend at the high-mass end within uncertainties. Passive galaxies exhibit a flatter mass–size relation, and their typical size (intercept at fixed stellar mass) lies between that of the coeval field and cluster passive populations, indicating an intermediate evolutionary stage. In both star-forming and passive systems, bulges are systematically more compact than discs. Galaxy sizes decrease slightly with increasing wavelength, whereas ALMA-detected dusty star-forming galaxies exhibit a much steeper wavelength dependence, consistent with centrally concentrated, obscured star formation. The passive fraction depends primarily on local density: it resembles the field at $\mathrm{\Sigma }\lesssim 100$–$200\mathrm{g}\mathrm{a}\mathrm{l}\mathrm{M}\mathrm{p}{\mathrm{c}}^{-2}$ and rises to $\sim 60\mathrm{\%}$ at $\mathrm{\Sigma }\gtrsim 1000\mathrm{g}\mathrm{a}\mathrm{l}\mathrm{M}\mathrm{p}{\mathrm{c}}^{-2}$, although with relatively large uncertainties due to the small number of galaxies in the highest-density bins, with no significant additional dependence on clustercentric distance. We also find a weak but significant correlation between local density and Sérsic index (strongest in F410M), but no clear correlation with effective radius or the star-formation rate surface density ${\mathrm{\Sigma }}_{\mathrm{S}\mathrm{F}\mathrm{R}}$. These results support a picture in which quenching and structural transformation are already advanced, while the size growth is still ongoing.