A new framework for understanding the evolution of early-type galaxies

Context. We have recently suggested that the combination of the scalar virial theorem (M-s proportional to R-e sigma(2)) and the L = L-0'sigma(beta) law, with L-0' and beta changing from galaxy to galaxy (and with time), can provide a new set of equations valid for investigating the evolution of early-type galaxies. These equations are able to account for the tilt of the fundamental plane and to explain the observed distributions of early-type galaxies in all its projections. Aims. In this paper we analyze the advantages offered by these equations, derive the beta and L-0' parameters for real and simulated galaxies, and demonstrate that depending on the value of beta galaxies can move only along some permitted directions in the fundamental plane projections. Then we show that simple galaxy models that grow in mass by infall of gas and form stars with a star formation rate depending on the stellar velocity dispersion nicely reproduce the observed distributions of early-type galaxies in the fundamental plane projections and yield beta s that agree with the measured values. Methods. We derive the mutual relationships among the stellar mass, e ffective radius, velocity dispersion, and luminosity of earlytype galaxies as a function of beta and calculate the coe fficients of the fundamental plane. Then, using the simple infall models, we show that the star formation history of early-type galaxies is compatible with the sigma-dependent star formation rate, and that both positive and negative values of beta are possible in a standard theory of galaxy evolution. Results. The parameter beta(t) offers a new view of the evolution of early-type galaxies. In brief, it gives a coherent interpretation of the fundamental plane and of the motions of galaxies in its projections; it is the fingerprint of their evolution; it measures the degree of virialization of early-type galaxies; and finally it allows us to infer their evolution in the near past.