2005ApJ...620L..83H

Using deep Hubble Space Telescope Advanced Camera for Surveys imaging and Very Large Telescope FOcal Reducer/low dispersion Spectrograph 2 spectra, we determined the velocity dispersions, effective radii, and surface brightnesses for four early-type galaxies in the z=1.237 cluster RDCS 1252.9-2927. All four galaxies are massive, greater than 10¹¹ M_☉. These four galaxies, combined with three from RDCS 0848+4453 at z=1.276, establish the fundamental plane of massive early-type cluster galaxies at z{d1}=1.25. The offset of the fundamental plane shows that the luminosity evolution in rest-frame B is ΔlnM/L_B=(-0.98±0.06)z for galaxies with M>10^11.5 M_☉. To reproduce the observed mass-to-light ratio (M/L) evolution, we determine the characteristic age of the stars in these M>10^11.5 M_☉galaxies to be 3.0^+0.3_–0.3Gyr; i.e., z_*=3.4^+0.5_–0.4. Including selection effects caused by morphological bias (the ``progenitor bias''), we estimate an age of 2.1^+0.2_–0.2Gyr, or z_*=2.3^+0.2_–0.2for the elliptical galaxy population. Massive cluster early-type galaxies appear to have a large fraction of stars that formed early in the history of the universe. However, there is a large scatter in the derived M/L values, which is confirmed by the spread in the galaxies' colors. Two lower mass galaxies in our z{d1}=1.25 sample have much lower M/L values, implying significant star formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, M≃10¹¹ M_☉, galaxies at z≃1.5.