Forming Early-Type Galaxies in LambdaCDM Simulations -I. Assembly
  histories
  We present a sample of nine high resolution cosmological simulations in the
mass range of M_vir=7x10^11-4x10^12 M_sun starting from LambdaCDM initial
conditions. Our simulations include primordial radiative cooling,
photoionization, star formation, supernova II feedback, but exclude supernova
driven winds and AGN 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 towards 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 (gamma'~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.
