2006ApJ...639L...9J

We present the fundamental plane (FP) for 38 early-type galaxies in the two rich galaxy clusters RX J0152.7-1357 (z=0.83) and RX J1226.9+3332 (z=0.89), reaching a limiting magnitude of M_B=-19.8 in the rest frame of the clusters. While the zero-point offset of the FP for these high-redshift clusters relative to our low-redshift sample is consistent with passive evolution with a formation redshift of z_form~3.2, the FP for the high-redshift clusters is not only shifted as expected for a mass-independent z_formbut rotated relative to the low-redshift sample. Expressed as a relation between the galaxy masses and the mass-to-light ratios, the FP is significantly steeper for the high-redshift clusters than for our low-redshift sample. We interpret this as a mass dependency of the star formation history, as has been suggested by other recent studies. The low-mass galaxies (10^10.3 M_☉) have experienced star formation as recently as z~1.35 (1.5 Gyr prior to their look-back time), while galaxies with masses larger than 10^11.3 M_☉had their last major star formation episode at z>4.5.