The stellar halos of ETGs in the IllustrisTNG simulations: the
  photometric and kinematic diversity of galaxies at large radii
  We characterize the photometric and kinematic properties of simulated
early-type galaxy (ETG) stellar halos, and compare them to observations. We
select a sample of ~1200 ETGs in the TNG100 and TNG50 simulations, spanning a
stellar mass range of $10^{10.3}-10^{12}M_{\odot}$ and within the range of
(g-r) colour and lambda-ellipticity diagram populated by observed ETGs. We
determine photometric parameters, intrinsic shapes, and kinematic observables
in their extended stellar halos. We study the variation in kinematics from
center to halo and connect it to a change in the intrinsic shape of the
galaxies. We find that the simulated galaxy sample reproduces the diversity of
kinematic properties observed in ETG halos. Simulated fast rotators (FRs)
divide almost evenly in one third having flat lambda profiles and high halo
rotational support, a third with gently decreasing profiles, and another third
with low halo rotation. Slow rotators (SRs) tend to have increased rotation in
the outskirts, with half of them exceeding lambda=0.2. For
$M_{*}>10^{11.5}M_{\odot}$ halo rotation is unimportant. A similar variety of
properties is found for the stellar halo intrinsic shapes. Rotational support
and shape are deeply related: the kinematic transition to lower rotational
support is accompanied by a change towards rounder intrinsic shape. Triaxiality
in the halos of FRs increases outwards and with stellar mass. Simulated SRs
have relatively constant triaxiality profiles. Simulated stellar halos show a
large variety of structural properties, with quantitative but no clear
qualitative differences between FRs and SRs. At the same stellar mass, stellar
halo properties show a gradual transition and significant overlap between the
two families, despite the clear bimodality in the central regions. This is in
agreement with observations of extended photometry and kinematics. [abridged]
