Extreme kinematic misalignment in IllustrisTNG galaxies: the origin,
  structure and internal dynamics of galaxies with a large-scale
  counterrotation
  Modern galaxy formation theory suggests that the misalignment between stellar
and gaseous components usually results from an external gas accretion and/or
interaction with other galaxies. The extreme case of the kinematic misalignment
is demonstrated by so-called galaxies with counterrotation that possess two
distinct components rotating in opposite directions with respect to each other.
We provide an in-deep analysis of galaxies with counterrotation from
IllustrisTNG100 cosmological simulations. We have found $25$ galaxies with
substantial stellar counterrotation in the stellar mass range of
$2\times10^{9}-3\times10^{10}$~\Msun. In our sample the stellar counterrotation
is a result of an external gas infall happened $\approx 2-8$~Gyr ago. The
infall leads to the initial removal of pre-existing gas, which is captured and
mixed together with the infalling component. The gas mixture ends up in the
counterrotating gaseous disc. We show that $\approx 90\%$ of the stellar
counterrotation formed in-situ, in the counterrotating gas. During the early
phases of the infall, gas can be found in inclined extended and rather thin
disc-like structures, and in some galaxies they are similar to (nearly-)~polar
disc or ring-like structures. We discuss a possible link between the gas
infall, AGN activity and the formation of misaligned components. In particular,
we suggest that the AGN activity does not cause the counterrotation, although
it is efficiently triggered by the retrograde gas infall, and it correlates
well with the misaligned component appearance. We also find evidence of the
stellar disc heating visible as an increase of the vertical-to-radial velocity
dispersion ratio above unity in both co- and counterrotating components, which
implies the importance of the kinematical misalignment in shaping the velocity
ellipsoids in disc galaxies.
