Friday, February 17, 2012

1202.3441 (Peter H. Johansson et al.)

Forming Early-Type Galaxies in LambdaCDM Simulations -I. Assembly histories    [PDF]

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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