Formation channels of slowly rotating early-type galaxies
  We study the evidence for a diversity of formation processes in early-type
galaxies by presenting the first complete volume-limited sample of slow
rotators with both integral-field kinematics from the ATLAS3D Project and high
spatial resolution photometry from the Hubble Space Telescope. Analysing the
nuclear surface brightness profiles of 12 newly imaged slow rotators, we
classify their light profiles as core-less, and place an upper limit to the
core size of ~10 pc. Considering the full magnitude and volume-limited ATLAS3D
sample, we correlate the presence or lack of cores with stellar kinematics,
including the proxy for the stellar angular momentum and the velocity
dispersion within one half-light radius, stellar mass, stellar age,
$\alpha$-element abundance, and age and metallicity gradients. More than half
of the slow rotators have core-less light profiles, and they are all less
massive than $10^{11}$ Msun. Core-less slow rotators show evidence for
counter-rotating flattened structures, have steeper metallicity gradients, and
a larger dispersion of gradient values than core slow rotators. Our results
suggest that core and core-less slow rotators have different assembly
processes, where the former are the relics of massive dissipation-less merging
in the presence of central supermassive black holes. Formation processes of
core-less slow rotators are consistent with accretion of counter-rotating gas
or gas-rich mergers of special orbital configurations, which lower the final
net angular momentum of stars, but support star formation. We also highlight
core fast rotators as galaxies that share properties of core slow rotators and
core-less slow rotators. Formation processes similar to those for core-less
slow rotators can be invoked to explain the assembly of core fast rotators,
with the distinction that these processes form or preserve cores.[Abridged]
