The Fundamenal Plane in CL0024 at z=0.4: Implications for the Evolution of the Mass-to-Light Ratio

We present results on the Fundamental Plane of early-type galaxies in the rich cluster CL 0024+16 at z=0.391. The galaxies satisfy a tight Fundamental Plane relation which is similar to that at low redshift. The scatter is 15 % in log re. The data show that massive early-type galaxies existed at z=0.4. The evolution of the M/L ratio is derived from the Fundamental Plane, by a comparison with Coma. The M/L ratio increases by 31 +- 12 % between z=0.391, and z=0.023. The evolution is low when compared to models for stellar populations. The expected evolution depends on the IMF, q0, and the formation redshift of the galaxies. The data are in agreement with high formation redshifts. The most serious bias in the modeling may be the progenitor bias: if the progenitors of some current day early-types are spirals at z=0.4, they would not be included in the sample, and the sample would be biased towards the oldest galaxies. More data are needed to measure the evolution of the Fundamental Plane more precisely, and its scatter. There is a hint that the form of the Fundamental Plane changes with redshift, and this needs to be determined better.