The stellar halos of ETGs in the IllustrisTNG simulations: II.
  Accretion, merger history, and dark halo connection
  Stellar halos in early-type galaxies (ETGs) are shaped by their accretion and
merger histories. We use a sample of 1114 ETGs in the TNG100 simulation with
stellar masses $10^{10.3}\leq M_{*}/M_\odot\leq 10^{12}$, selected at z=0
within the range of g-r colour and lambda-ellipticity diagram populated by
observed ETGs. We study how the rotational support and intrinsic shapes of the
stellar halos depend on the fraction of accreted stars, overall and separately
by major, minor, and mini mergers. Accretion histories in TNG100 ETGs as well
as the radial distributions of ex-situ stars $f_{ex}(R)$ strongly correlate
with stellar mass. Low-mass ETGs have characteristic peaked rotation profiles
and near-oblate shapes with rounder halos that are completely driven by the
in-situ stars. At high $f_{ex}$ major mergers decrease the in-situ peak in
rotation velocity, flatten the $V_{*}/\sigma_{*}(R)$ profiles, and increase the
triaxiality of the stellar halos. Kinematic transition radii do not trace the
transition between in-situ and ex-situ dominated regions, but for systems with
$M_{*}>10^{10.6}M_\odot$ the local rotational support decreases with the local
ex-situ fraction $f_{ex}(R)$ and triaxiality increases with $f_{ex}$. These
correlations are followed by fast and slow rotators alike with a continuous and
overlapping sequence of properties. Merger events dynamically couple stars and
dark matter: in high mass ETGs and at large radii where $f_{ex}\gtrsim0.5$,
both components tend to have similar intrinsic shapes and rotational support,
and nearly aligned principal axes and spin directions. Based on these results
we suggest that extended photometry and kinematics of massive ETGs
($M_{*}>10^{10.6}M_\odot$) can be used to estimate the local fraction of
ex-situ stars and to approximate the intrinsic shapes and rotational support of
the co-spatial dark matter component. [abridged]
