Understanding the Structural Scaling Relations of Early-Type Galaxies
  We use a large suite of hydrodynamical simulations of binary galaxy mergers
to construct and calibrate a physical prescription for computing the effective
radii and velocity dispersions of spheroids. We implement this prescription
within a semi-analytic model embedded in merger trees extracted from the
Bolshoi Lambda-CDM N-body simulation, accounting for spheroid growth via major
and minor mergers as well as disk instabilities. We find that without disk
instabilities, our model does not predict sufficient numbers of intermediate
mass early-type galaxies in the local universe. Spheroids also form earlier in
models with spheroid growth via disk instabilities. Our model correctly
predicts the normalization, slope, and scatter of the low-redshift size-mass
and Fundamental Plane relations for early type galaxies. It predicts a degree
of curvature in the Faber-Jackson relation that is not seen in local
observations, but this could be alleviated if higher mass spheroids have more
bottom-heavy initial mass functions. The model also correctly predicts the
observed strong evolution of the size-mass relation for spheroids out to higher
redshifts, as well as the slower evolution in the normalization of the
Faber-Jackson relation. We emphasize that these are genuine predictions of the
model since it was tuned to match hydrodynamical simulations and not these
observations.
