Using a high resolution hydrodynamical cosmological simulation of the formation of a massive spheroidal galaxy we show that elliptical galaxies can be very compact and massive at high redshift in agreement with recent observations. Accretion of stripped in-falling stellar material increases the size of the system with time and the central concentration is reduced by dynamical friction of the surviving stellar cores. In a specific case of a spheroidal galaxy with a final stellar mass of 1.5 × 10 11 M ⊙ we find that the effective radius r e increases from 0.7 ± 0.2 kpc at z = 3 to r e = 2.4 ± 0.4 kpc at z = 0 with a concomitant decrease in the effective density of an order of magnitude and a decrease of the central velocity dispersion by approximately 20% over this time interval. A simple argument based on the virial theorem shows that during the accretion of weakly bound material (minor mergers) the radius can increase as the square of the mass in contrast to the usual linear rate of increase for major mergers. By undergoing minor mergers compact high redshift spheroids can evolve into present-day systems with sizes and concentrations similar to observed local ellipticals. This indicates that minor mergers may be the main driver for the late evolution of sizes and densities of early-type galaxies.

Cosmological simulations of galaxy formation appear to show a 'two-phase' character with a rapid early phase at z 2 during which 'in-situ' stars are formed within the galaxy from infalling cold gas followed by an extended phase since z 3 during which 'ex-situ' stars are primarily accreted. In the latter phase massive systems grow considerably in mass and radius by accretion of smaller satellite stellar systems formed at quite early times (z > 3) outside of the virial radius of the forming central galaxy. These tentative conclusions are obtained from high resolution re-simulations of 39 individual galaxies in a full cosmological context with present-day virial halo masses ranging from 7×10 11 M ⊙ h −1 M vir 2.7 × 10 13 M ⊙ h −1 (h=0.72) and central galaxy masses between 4.5 × 10 10 M ⊙ h −1 M * 3.6 × 10 11 M ⊙ h −1 . The simulations include the effects of a uniform UV background, radiative cooling, star formation and energetic feedback from SNII. The importance of stellar accretion increases with galaxy mass and towards lower redshift. In our simulations lower mass galaxies (M * 9×10 10 M ⊙ h −1 ) accrete about 60 per cent of their present-day stellar mass. High mass galaxy (M * 1.7×10 11 M ⊙ h −1 ) assembly is dominated by accretion and merging with about 80 per cent of the stars added by the present-day. In general the simulated galaxies approximately double their mass since z=1. For massive systems this mass growth is not accompanied by significant star formation. The majority of the in-situ created stars is formed at z > 2, primarily out of cold gas flows. We recover the observational result of 'archaeological downsizing', where the most massive galaxies harbor the oldest stars. We find that this is not in contradiction with hierarchical structure formation. Most stars in the massive galaxies are formed early on in smaller structures, the galaxies themselves are assembled late.

We describe high-resolution smoothed particle hydrodynamics (SPH) simulations of three approximately M Ã field galaxies starting from ÃCDM initial conditions. The simulations are made intentionally simple, and include photoionization, cooling of the intergalactic medium, and star formation, but not feedback from AGNs or supernovae. All of the galaxies undergo an initial burst of star formation at z % 5, accompanied by the formation of a bubble of heated gas. Two out of three galaxies show early-type properties at present, whereas only one of them experienced a major merger. Heating from shocks and PdV work dominates over cooling so that for most of the gas the temperature is an increasing function of time. By z % 1 a significant fraction of the final stellar system is in place and the spectral energy distribution resembles those of observed massive red galaxies. The galaxies have grown from z ¼ 1 ! 0 on average by 25% in mass and in size by gas-poor (dry) stellar mergers. By the present day the simulated galaxies are old (%10 Gyr), kinematically hot stellar systems surrounded by hot gaseous haloes. Stars dominate the mass of the galaxies up to %4 effective radii (%10 kpc). Kinematic and most photometric properties are in good agreement with those of observed elliptical galaxies. The galaxy with a major merger develops a counter-rotating core. Our simulations show that realistic intermediate-mass giant elliptical galaxies with plausible formation histories can be formed from ÃCDM initial conditions even without requiring recent major mergers or feedback from supernovae or AGNs.

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 11 M ⊙ (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 R 1/2 ∝ (1 + z) α with α = −1.44 for quiescent galaxies (α = −1.12 for all galaxies). The velocity dispersions drop by about one-third since z ≈ 2 , following σ 1/2 ∝ (1 + z) β with β = 0.44 for the quiescent galaxies (β = 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. We therefore conclude that the evolution of massive early-type galaxies since z ≈ 2 and their present-day properties are predominantly determined by frequent 'minor' mergers of moderate mass-ratios and not by major mergers alone.