The Fundamental Plane in CL0024 at z = 0.4: Implications for the evolution of the mass-to-light ratio

Abstract

We present results on the Fundamental Plane of early-type galaxies in the rich cluster CL 0024+16 at z=0.39. The internal velocity dispersions of the galaxies have been measured from a 19-h integration at the MMT. The photometric parameters of the galaxies have been measured from deep HST images. The galaxies satisfy a tight Fundamental Plane relation which is similar to that at low redshift. The scatter is 15 per cent in log re, also very similar to that at low redshift. The data show that massive early-type galaxies existed at z=0.4, and extend earlier studies of the luminosities and colours of early-types in rich clusters. 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 per cent between z = 0.39 and z = 0.023. The evolution is low when compared with models for stellar populations. The expected evolution depends on the IMF, go, and the formation redshift of the galaxies. The data are in agreement with high formation redshifts. The modelling is still uncertain, however, because of various possible biases. The most serious bias may be the progenitor bias: if the progenitors of some current-day early-types were 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. Deeper samples on more clusters would be valuable. Studies of the richest nearby clusters may help to test the underlying hypothesis that the Fundamental Plane is identical in all clusters.