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 10^10 M_ 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 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_* 10^11M_ (11 out of 40) at z=2 are compact with projected half-mass radii of 0. 77 (0. 24) kpc and line-of-sight velocity dispersions within the projected half-mass radius of 262 (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 5-6, following R₁/₂ (1+z) ^ with = -1. 44 for quiescent galaxies (= -1. 12 for all galaxies). The velocity dispersions drop by about one-third since z 2, following ₁/₂ (1+z) ^ with = 0. 44 for the quiescent galaxies (= 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 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)