The evolution of early-type galaxies at z ∼ 1 from the K20 survey

We have performed VLT spectroscopy of an almost complete sample of 18 early-type galaxies with 0.88 <= z <= 1.3 plus two at z = 0.67, selected from the K20 survey, and derived the velocity dispersion for 15+2 of them. By combining these data with HST and VLT images, we study the Fundamental Plane (FP), the Faber - Jackson and the Kormendy relations at z similar to 1. Compared with the local one, the FP at z similar to 1 has a remarkably small scatter, and shows both an offset and a rotation, which we interpret in terms of evolution of the mass-to-light ratio, and possibly of the size. We give evidence that the evolution rate depends on galaxy mass, being faster for less massive galaxies. We discuss the possible factors driving the evolution of spheroids and compare our results with the predictions of the hierachical models of galaxy formation.