UC Santa Cruz

We construct a simple prescription based on hydrodynamical simulations to predict the effective radii and velocity dispersions of early-type galaxies. We apply this prescription to a semi-analytic model, allowing for bulge formation via major and minor mergers as well as disk instabilities. We find that it is necessary to include disk instabilities in the model to reproduce the mass function of early-type galaxies, and that galaxies that form bulges via disk instability tend to form bulges earlier. This model is able to reproduce the low-redshift size-mass and Fundamental Plane relations. It predicts a degree of curvature in the Faber-Jackson relation that is not seen in local observations, but this could be offset if higher mass spheroids have more bottom-heavy initial mass functions. The model is also able to match the observed rapid evolution of the size-mass relation out to higher redshifts, as well as the slower evolution in the normalization of the Faber-Jackson relation.