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 of cluster spheroids .
We find that the FP at z \sim 1.3 is offset and rotated ( \sim 3 \sigma ) with respect to the local FP .
The offset corresponds to a mean evolution \Delta log ( M _ { dyn } /L _ { B } ) = ( -0.5 \pm 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 _ { \odot } ) \geq 11.5 ) have a higher-formation redshift ( z _ { f } \geq 6.5 ) than lower-mass ones ( z _ { f } \leq 2 for log ( M _ { dyn } /M _ { \odot } \leq 10 ) ) , with a median z _ { f } \simeq 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 IMF , 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 _ { \odot } galaxies , while accounts for \sim 85 % of the evolution at M _ { dyn } < 10 ^ { 11 } M _ { \odot } .
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 _ { \odot } .
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 .