The Fundamental Plane of cluster spheroidal galaxies at z ∼ 1.3: evidence for mass-dependent evolution

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 similar to 1.3 is offset and rotated (similar to 3 sigma) with respect to the local FP. The offset corresponds to a mean evolution Delta log(M-dyn/L-B)=(-0.5 +/- 0.1)z. High-redshift galaxies follow a steeper mass-dependent M-dyn/L-B-M-dyn relation than local ones. Assuming Delta log(M-dyn/L-B) = Delta log(M*/L-B), higher mass galaxies [log(M-dyn/M-circle dot) >= 11.5] have a higher formation redshift (z(f) >= 6.5) than lower mass ones [z(f) <= 2 for log(M-dyn/M-circle dot <= 10)], with a median z(f) similar or equal to 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, M-dyn/M* varies systematically with mass and mass density. It follows that the evolution of the stellar populations (M*/L-B) accounts for the observed evolution of M-dyn/L-B for M-dyn > 10(11) M-circle dot galaxies, while accounts for similar to 85 per cent of the evolution at M-dyn < 10(11) M-circle dot. 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 Delta log(M-dyn/L-B) and Delta log(M*/L-B) at masses <10(11) M-circle dot. 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.