The cosmological size and velocity dispersion evolution of massive
  early-type galaxies
  We analyze 40 cosmological re-simulations of individual massive galaxies with
present-day stellar masses of $M_{*} > 6.3 \times 10^{10} M_{\odot}$ in order
to investigate the physical origin of the observed strong increase in galaxy
sizes and the decrease of the stellar velocity dispersions since redshift $z
\approx 2$. At present 25 out of 40 galaxies are quiescent with structural
parameters (sizes and velocity dispersions) in agreement with local early type
galaxies. At z=2 all simulated galaxies with $M_* \gtrsim 10^{11}M_{\odot}$ (11
out of 40) at z=2 are compact with projected half-mass radii of $\approx$ 0.77
($\pm$0.24) kpc and line-of-sight velocity dispersions within the projected
half-mass radius of $\approx$ 262 ($\pm$28) kms$^{-1}$ (3 out of 11 are already
quiescent). Similar to observed compact early-type galaxies at high redshift
the simulated galaxies are clearly offset from the local mass-size and
mass-velocity dispersion relations. Towards redshift zero the sizes increase by
a factor of $\sim 5-6$, following $R_{1/2} \propto (1+z)^{\alpha}$ with $\alpha
= -1.44$ for quiescent galaxies ($\alpha = -1.12$ for all galaxies). The
velocity dispersions drop by about one-third since $z \approx 2$, following
$\sigma_{1/2} \propto (1+z)^{\beta}$ with $\beta = 0.44$ for the quiescent
galaxies ($\beta = 0.37$ for all galaxies). The simulated size and dispersion
evolution is in good agreement with observations and results from the
subsequent accretion and merging of stellar systems at $z\lesssim 2$ which is a
natural consequence of the hierarchical structure formation. A significant
number of the simulated massive galaxies (7 out of 40) experience no merger
more massive than 1:4 (usually considered as major mergers). On average, the
dominant accretion mode is stellar minor mergers with a mass-weighted
mass-ratio of 1:5. (abridged)
