2003ApJ...585...78V

We present results on the fundamental plane of early-type galaxies in the cluster RDCS J0848+4453 at z=1.27. Internal velocity dispersions of three K-selected early-type galaxies are determined from deep Keck spectra, using absorption lines in the rest-frame wavelength range 3400-4000 Å. Structural parameters are determined from Hubble Space Telescope NICMOS images. The galaxies show substantial offsets from the fundamental plane of the nearby Coma Cluster, as expected from passive evolution of their stellar populations. The offsets from the fundamental plane can be expressed as offsets in mass-to-light (M/L) ratio. The M/L ratios of the two most massive galaxies are consistent with an extrapolation of results obtained for clusters at 0.02<z<0.83. The evolution of early-type galaxies with masses greater than 10¹¹ M_☉is well described by lnM/L_B∝(-1.06±0.09)z, corresponding to passive evolution of -1.50±0.13 mag at z=1.3. Ignoring selection effects, the best-fitting stellar formation redshift is z_*=2.6^+0.9_–0.4for an Ω_m=0.3, Ω_Λ=0.7 cosmology and a Salpeter initial mass function, corresponding to a luminosity-weighted age at the epoch of observation of ∼2 Gyr. The M/L ratios of these two galaxies are also in excellent agreement with predictions from models that include selection effects caused by morphological evolution (``progenitor bias''). The third galaxy is a factor of ∼10 less massive than the other two, shows strong Balmer absorption lines in its spectrum, and is offset from the Coma fundamental plane by 2.9 mag in rest-frame B. Larger samples are required to determine whether such young early-type galaxies are common in high-redshift clusters. Despite their large range in M/L ratios, all three galaxies fall in the ``extremely red object'' class with I-H>3 and R-K>5, and our results show that it is hazardous to use simple models for converting luminosity to mass for these objects. The work presented here, and previous mass measurements at lower redshift, can be considered first steps to empirically disentangle luminosity and mass evolution at the high-mass end of the galaxy population, lifting an important degeneracy in the interpretation of evolution of the luminosity function.