Modelling the ages and metallicities of early-type galaxies in Fundamental Plane space

Recent observations have probed the formation histories of nearby elliptical galaxies by tracking correlations between the stellar population parameters, age and metallicity, and the structural parameters that enter the Fundamental Plane, size R-e, and velocity dispersion sigma. These studies have found intriguing correlations between these four parameters. In this work, we make use of a semi-analytic model, based on halo merger trees extracted from the Bolshoi cosmological simulation, that predicts the structural properties of spheroid-dominated galaxies based on an analytic model that has been tested and calibrated against an extensive suite of hydrodynamic+N-body binary merger simulations. We predict the R-e, sigma, luminosity, age, and metallicity of spheroid-dominated galaxies, enabling us to compare directly to observations. Our model predicts a strong correlation between age and sigma for early-type galaxies, and no significant correlation between age and radius, in agreement with observations. In addition, we predict a strong correlation between metallicity and sigma, and a weak correlation between metallicity and R-e, in qualitative agreement with observations. We find that the correlations with sigma arise as a result of the strong link between sigma and the galaxy's assembly time. Minor mergers produce a large change in radius while leaving sigma nearly the same, which explains the weaker trends with radius.