Formation of Early-Type Galaxies from Cosmological Initial Conditions

We describe high-resolution smoothed particle hydrodynamics (SPH) simulations of three approximately M* field galaxies starting from ΛCDM 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 = 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 ΛCDM initial conditions even without requiring recent major mergers or feedback from supernovae or AGNs.