Peter H. Johansson, Thorsten Naab, Jeremiah P. Ostriker
We present a sample of nine very high resolution cosmological simulations
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. We confirm our earlier results
with higher resolution simulations and demonstrate that the simulated galaxies
assemble in two phases, with the initial growth dominated by compact in situ
star formation fueled by cold, low entropy gas streams, whereas the late growth
is dominated by accretion of old stars formed in subunits outside the main
galaxy. The two-phase formation mechanism naturally explains the observed
downsizing, bimodality and size growth of the galaxy population. Very high
resolution simulations show that gravitational feedback strongly suppresses
late star formation in massive galaxies contributing to the observed galaxy
color bimodality. However, additional heating sources probably in the form of
AGN and SNI feedback are also required to prevent late gas inflows and
associated residual star formation in the more massive galaxies. The accretion
of stellar material (dry minor mergers) is also responsible for the observed
size growth of early-type galaxies. Consistent with their assembly histories 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. In addition, the logarithmic slope of the total density
profile is nearly isothermal at the present-day (gamma'~1.9-2.2) also in good
agreement with recent lensing observations. Our simulations predict almost
constant slopes until redshift z =1 and then steeper slopes of gamma~3 at
higher redshifts. (Abridged)
View original:
http://arxiv.org/abs/1202.3441
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