Forming Early-type Galaxies in ΛCDM Simulations. I. Assembly Histories

We present a sample of nine high-resolution cosmological simulations in the mass range of M _vir = 7 × 10¹¹-4 × 10¹² M _⊙ starting from ΛCDM initial conditions. Our simulations include primordial radiative cooling, photoionization, star formation, supernova II feedback, but exclude supernova-driven winds and active galactic nucleus feedback. The simulated galaxies assemble in two phases, with the initial growth dominated by compact (r < r _eff) in situ star formation fueled by cold, low-entropy gas streams resulting in a very similar mean assembly redshift of z _{f, ins} ~ 2.5 for the in situ stellar component in all galaxies. The late growth is dominated by accretion of old stars formed in subunits outside the main galaxy (r > r _eff) resulting in an assembly redshift of z _{f, acc} ~ 0.5-1.5 with much larger scatter. We find a positive correlation between the fraction of accreted stars and the final mass of our galaxies. We show that gravitational feedback strongly suppresses late star formation in massive galaxies contributing to the observed galaxy color bimodality. The accretion of stellar material is also responsible for the observed size growth of early-type galaxies. In addition, we find that the dark matter fractions within the stellar half-mass radii continuously increase toward lower redshift from about f _DM ~ 0.05 at z ~ 3 to f _DM ~ 0.1-0.3 at z = 0. Furthermore, the logarithmic slope of the total density profile is nearly isothermal at the present day (γ' ~ 1.9-2.2). Finally, the input of gravitational heating lowers the central dark matter densities in the galaxies, with the effect being smaller compared to simulations without supernova feedback.