Modeling 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, radius 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 radius, 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 radius,
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
assembly time. Minor mergers produce a large change in radius while leaving
sigma nearly the same, which explains the weaker trends with radius.
