2020MNRAS.491.1777S

We present spectroscopic observations obtained at the Large Binocular Telescope in the field of the cluster XLSSJ0223-0436 at z = 1.22. We confirm 12 spheroids cluster members and determine stellar velocity dispersion for 7 of them. We combine these data with those in the literature for clusters RXJ0848+4453 at z = 1.27 (8 galaxies) and XMMJ2235-2557 at z = 1.39 (7 galaxies) to determine the Fundamental Plane (FP) of cluster spheroids. We find that the FP at z ∼ 1.3 is offset and rotated (∼3σ) with respect to the local FP. The offset corresponds to a mean evolution [?]log(Mdyn/LB) = (-0.5 ± 0.1)z. High-redshift galaxies follow a steeper mass-dependent Mdyn/LB-Mdyn relation than local ones. Assuming [?] log(Mdyn/LB) = [?] log(M*/LB), higher mass galaxies [log(Mdyn/M☉) >= 11.5] have a higher formation redshift (zf >= 6.5) than lower mass ones [zf <=2 for log(Mdyn/M☉ <= 10)], with a median zf ≃ 2.5 for the whole sample. Also, galaxies with higher stellar mass density host stellar populations formed earlier than those in lower density galaxies. At fixed initial mass function, Mdyn/M* varies systematically with mass and mass density. It follows that the evolution of the stellar populations (M*/LB) accounts for the observed evolution of Mdyn/LB for Mdyn > 1011 M☉ galaxies, while accounts for ∼85 per cent of the evolution at Mdyn < 1011 M☉. We find no evidence in favour of structural evolution of individual galaxies, while we find evidences that spheroids later added to the population may account for the observed discrepancy between [?]log(Mdyn/LB) and [?] log(M*/LB) at masses <1011 M☉. Thus, the evolution of the FP of cluster spheroids is consistent with the mass-dependent and mass density-dependent evolution of their stellar populations superimposed to a minor contribution of spheroids joining the population at later times.