Early-type galaxy density profiles from IllustrisTNG: III. Effects on
  outer kinematic structure
  Early-type galaxies (ETGs) possess total radial density profiles that are
largely described by singular isothermal spheres, which can lead to
non-Gaussian line-of-sight velocity dispersion (LOSVD) under anisotropic
stellar orbits. However, recent observations of local ETGs in the MASSIVE
Survey reveal outer kinematic structures at $1.5 R_{eff}$ that are inconsistent
with fixed isothermal density profiles; the authors proposed varying density
profiles as an explanation. We aim to verify this conjecture and understand the
influence of stellar assembly on the formation of these non-canonical kinematic
features through mock ETGs with well-studied density profiles in IllustrisTNG.
We create 2D maps mimicking integral-field-unit (IFU) observations to extract
projected stellar kinematic features for 207 ETGs in the TNG100-1 box with
stellar mass $M_{\ast} > 10^{11} M_{\odot}$. The mock observations reproduce
the key outer ($1.5R_{eff}$) kinematic structures in the MASSIVE ETGs,
including the puzzling positive correlation between velocity dispersion profile
outer slope $\gamma_{outer}$ and the kurtosis $h_4$'s gradient. We find that
$h_4$ is uncorrelated with stellar orbital anisotropy beyond $R_{eff}$; instead
we find that the variations in $\gamma_{outer}$ and outer $h_4$ (which
dominates the $h_4$ gradient) are both driven by variations of the density
profile at the outskirts across different ETGs. These findings corroborate the
proposed conjecture and rule out velocity anisotropy as the dominant driver of
non-Gaussian outer kinematic structure in ETGs. We also find that the outer
kurtosis and anisotropy correlate with different stellar assembly components,
with the former related to minor mergers or flyby interactions while the latter
is mainly driven by major mergers, suggesting distinct stellar assembly origins
that decorrelates these two properties.
