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Modern galaxy formation theory suggests that the misalignment between stellarand gaseous components usually results from an external gas accretion and/orinteraction with other galaxies. The extreme case of the kinematic misalignmentis demonstrated by so-called galaxies with counterrotation that possess twodistinct components rotating in opposite directions with respect to each other.We provide an in-deep analysis of galaxies with counterrotation fromIllustrisTNG100 cosmological simulations. We have found $25$ galaxies withsubstantial stellar counterrotation in the stellar mass range of$2\times10^{9}-3\times10^{10}$~\Msun. In our sample the stellar counterrotationis a result of an external gas infall happened $\approx 2-8$~Gyr ago. Theinfall leads to the initial removal of pre-existing gas, which is captured andmixed together with the infalling component. The gas mixture ends up in thecounterrotating gaseous disc. We show that $\approx 90\%$ of the stellarcounterrotation formed in-situ, in the counterrotating gas. During the earlyphases of the infall, gas can be found in inclined extended and rather thindisc-like structures, and in some galaxies they are similar to (nearly-)~polardisc or ring-like structures. We discuss a possible link between the gasinfall, AGN activity and the formation of misaligned components. In particular,we suggest that the AGN activity does not cause the counterrotation, althoughit is efficiently triggered by the retrograde gas infall, and it correlateswell with the misaligned component appearance. We also find evidence of thestellar disc heating visible as an increase of the vertical-to-radial velocitydispersion ratio above unity in both co- and counterrotating components, whichimplies the importance of the kinematical misalignment in shaping the velocityellipsoids in disc galaxies.

Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure and internal dynamics of galaxies with a large-scale counterrotation