We present new spatially-resolved Keck spectroscopy of early-type galaxies in three galaxy clusters at z \approx 0.5 .
In two companion papers ( van der Marel & van Dokkum 2006a , b ) we construct dynamical models of the galaxies and compare their modeled M / L ratios and dynamical structure to local samples .
Here we focus on the fundamental plane ( FP ) relation , and combine the kinematics with structural parameters determined from Hubble Space Telescope ( HST ) images .
The galaxies obey clear FP relations , which are offset from the FP of the nearby Coma cluster due to passive evolution of the stellar populations .
The z \approx 0.5 data are combined with published data for eleven additional clusters at 0.18 \leq z \leq 1.28 , to determine the evolution of the mean M / L _ { B } ratio of cluster galaxies with masses M \gtrsim 10 ^ { 11 } M _ { \odot } , as implied by the FP .
We find d \log ( M / L _ { B } ) / dz = -0.555 \pm 0.042 , stronger evolution than was previously inferred from smaller samples .
The observed evolution depends on the luminosity-weighted mean age of the stars in the galaxies , the initial mass function ( IMF ) , selection effects due to progenitor bias , and other parameters .
Assuming a normal IMF but allowing for various other sources of uncertainty , we find z _ { * } = 2.01 ^ { +0.22 } _ { -0.17 } for the luminosity-weighted mean star formation epoch .
The main uncertainty is the slope of the IMF in the range 1 - 2 M _ { \odot } : we find z _ { * } = 4.0 for a top-heavy IMF with slope x = 0 .
The M / L _ { B } ratios of the cluster galaxies are compared to those of field early-type galaxies at 0.32 \leq z \leq 1.14 .
Assuming that progenitor bias and the IMF do not depend on environment we find that the present-day age of stars in massive field galaxies is 4.1 % \pm 2.0 % ( \approx 0.4 Gyr ) less than that of stars in massive cluster galaxies .
This relatively small age difference is surprising in the context of expectations from “ standard ” hierarchical galaxy formation models , and provides a constraint on the physical processes that are responsible for halting star formation in the progenitors of today ’ s most massive galaxies .