Mergers in Galaxy Groups. II. The Fundamental Plane of Elliptical
  Galaxies
  Observations consistently show that elliptical galaxies follow a tight
"fundamental plane" scaling relation between size, mean surface brightness and
velocity dispersion, with the form $R \propto \sigma^{a}\mu^{b}$. This relation
not only has very small (<0.05 dex) intrinsic scatter, but also has
significantly different coefficients from the expected virial scaling (a
"tilt"). We analyze hundreds of simulations of elliptical galaxies formed from
mergers of spiral galaxies in groups to determine if the fundamental plane can
emerge from multiple, mostly minor and hierarchical collisionless mergers. We
find that these simulated ellipticals lie on a similar fundamental plane with
$a \approx 1.7$ and $b \approx 0.3$. The scatter about this plane is not larger
than observed, while the tilt is in the correct sense, although $a$ is larger
than for typical observations. This supports the idea that collisionless
mergers can contribute significantly to the tilt of the fundamental plane. The
tilt is mainly driven by a mass-dependent dark matter fraction, such that more
massive galaxies have larger dark matter fractions within $R_{e}$. We further
discuss the origin of this mass-dependent dark matter fraction and its
compatibility with strong lensing observations, as well as the links between
the fundamental plane, dynamical masses and the virial theorem.
