The Atlas3D Project - VI. Simulations of binary galaxy mergers and the
  link with Fast Rotators, Slow Rotators, and Kinematically Distinct Cores
  We study the formation of early-type galaxies through mergers with a sample
of 70 high-resolution (softening length < 60 pc and 12*10^6 particles)
numerical simulations of binary mergers of disc galaxies and 16 simulations of
ETG remergers. These simulations, designed to accompany observations and models
conducted within the Atlas3D project, encompass various mass ratios (from 1:1
to 6:1), initial conditions and orbital parameters. The progenitor disc
galaxies are spiral-like with bulge to disc ratios typical of Sb and Sc
galaxies. We find that binary mergers of disc galaxies with mass ratios of 3:1
and 6:1 are nearly always classified as Fast Rotators according to the Atlas3D
criterion (based on the lambda_R parameter): they preserve the structure of the
input fast rotating spiral progenitors. They have intrinsic ellipticities
larger than 0.5, cover intrinsic lambda_R values between 0.2 and 0.6, within
the range of observed Fast Rotators. Major disc mergers (mass ratios of 2:1 and
1:1) lead to both Fast and Slow Rotators. Most of the Fast Rotators produced in
major mergers have intermediate flattening, with ellipticities between 0.4 and
0.6. Most Slow Rotators formed in these binary disc mergers hold a stellar
Kinematically Distinct Core (KDC) in their 1-3 central kilo-parsec: these KDCs
are built from the stellar components of the progenitors. Besides a handful of
specific observed systems -- the counter-rotating discs (2-sigma galaxies) --
these therefore cannot reproduce the observed population of Slow Rotators in
the nearby Universe. The mass ratio of the progenitors is a fundamental
parameter for the formation of Slow Rotators in these binary mergers, but it
also requires a retrograde spin for the earlier-type (Sb) progenitor galaxy
with respect to the orbital angular momentum. (Abridged)
