IllustrisTNG insights: Factors affecting the presence of bars in disk galaxies

Aims. Bars are important in the secular evolution of galaxies. The reproduction of the fraction and size of bars is also regarded as an indicator for reproducing correct internal dynamical processes, which is a crucial test for cosmological simulations. This study aims to explore the reasons why some galaxies have bars at redshift z = 0 while others do not. Methods. We use ellipse fitting to measure the properties and evolution of bars in the IllustrisTNG cosmological simulation. By using the K–S two-sample test and tracing their evolutionary changes, we analyze the parameter differences between barred and unbarred galaxies. The properties of galaxies with short bars are also studied. Results. When tracing all disk galaxies at z = 0 back to z = 1, all of them show similar bar features at z = 1. The fraction of bars increases in barred and short-bar galaxies but decreases in unbarred galaxies during z = 1 − 0. In the case of disk galaxies with stellar mass log( M ∗ / M ⊙ ) > 10.8, nurture (mainly mergers) plays the most important role in suppressing or destroying bars. Bars are more likely to endure in galaxies that experience fewer mergers, which can be quantified by smaller stellar halos and ex situ mass fractions. Approximately 60% of the unbarred galaxies in the local Universe once had a bar. In contrast, the lack of responsiveness to bar instabilities (resulting in a larger Toomre- Q parameter) due to a less compact nature plays an important role in generating unbarred disk galaxies with stellar mass log( M ∗ / M ⊙ ) < 10.8. Moreover, short bars generally form at a similar time to normal bars, during which they either grow mildly or contract significantly. The fact that IllustrisTNG simulations produce too many galaxies with short bars indicates that the dynamical properties of the central regions in IllustrisTNG galaxies are less affected by external factors, such as mergers and gas inflows.