The ATLAS3D Project-- VIII: Modelling the Formation and Evolution of
  Fast and Slow Rotator Early-Type Galaxies within $\Lambda$CDM
  We propose a simple model for the origin of fast and slow rotator early-type
galaxies (ETG) within the hierarchical $\Lambda$CDM scenario, that is based on
the assumption that the mass fraction of stellar discs in ETGs is a proxy for
the specific angular momentum expressed via $\lambda_R$. Within our model we
reproduce the fraction of fast and slow rotators as a function of magnitude in
the \atl survey, assuming that fast rotating ETGs have at least 10% of their
total stellar mass in a disc component. In agreement with \atl observations we
find that slow rotators are predominantly galaxies with $ M_* > 10^{10.5}$
M$_{\odot}$ contributing $\sim 20%$ to the overall ETG population. We show in
detail that the growth histories of fast and slow rotators are different,
supporting the classification of ETGs into these two categories. Slow rotators
accrete between $\sim 50% -90%$ of their stellar mass from satellites and their
most massive progenitors have on average up to 3 major mergers during their
evolution. Fast rotators in contrast, accrete less than 50% and have on average
less than one major merger in their past.
  We find that the underlying physical reason for the different growth
histories is the slowing down and ultimately complete shut-down of gas cooling
in massive galaxies. Once cooling and associated star formation in disc stops,
galaxies grow via infall from satellites. Frequent minor mergers thereby,
destroy existing stellar discs via violent relaxation and also tend to lower
the specific angular momentum of the main stellar body, lowering $\lambda_R$
into the slow rotator regime. Abridged...
