Kinematic morphology of low-mass galaxies in IllustrisTNG

TL;DR

This study uses the TNG50-1 simulation to analyze the kinematic morphologies of low-mass galaxies, revealing that their rotational properties depend on cold gas disc alignment and are affected by numerical heating, with morphology reliability linked to stellar particle count.

Contribution

It provides new insights into the evolution of low-mass galaxy morphologies and assesses the impact of numerical effects and resolution on simulated galaxy properties.

Findings

Most low-mass galaxies are dispersion-dominated, consistent with observations.

Misalignment of cold gas discs leads to dispersion-dominated systems.

Numerical heating affects galaxy morphology, especially in central regions.

Abstract

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.