  The origin of diverse kinematic morphologies observed in low-mass galaxies is
unclear. In this study, we investigate the kinematic morphologies of central
galaxies with stellar mass $10^{8.5-9.0} M_\odot$ at $z=0$ in the TNG50-1
cosmological simulation. The majority of the low-mass galaxies in TNG50-1 are
dispersion-dominated, consistent with observations. By tracing the evolutionary
histories of simulated low-mass galaxies, we find that while most stars form in
rotating cold gas discs, the orientation of the star-forming discs relative to
the galaxies may evolve with cosmic time. If the cold gas disc remains aligning
with the galaxy during its evolution, stars formed at different times share the
same rotational direction, leading to a rotation-dominated system. On the
contrary, frequent misalignment of cold gas disc would result in a
dispersion-dominated system. In addition, we also find that the two-body
scattering can have a non-negligible numerical heating effect on the simulated
galaxy morphology, especially at central regions of galaxies and for relatively
low-mass galaxies. By comparing results of simulations with different
resolutions, our results suggest that the simulated morphology of galaxies is
roughly reliable when their number of stellar particles exceeds about $10^{4}$,
and bulge morphology of galaxies can not be resolved robustly at the resolution
level of TNG50-1.
