Origins and lifetimes of secular and tidal bars in simulated disc galaxies

TL;DR

This study uses cosmological simulations to analyze how stellar bars form in disc galaxies, revealing both internal secular processes and external tidal interactions as key mechanisms influencing bar formation and longevity.

Contribution

It introduces a new metric, $S_{bar}$, to quantify tidal influences, and distinguishes between secular and tidally induced bars, showing their different origins and host galaxy properties.

Findings

Most bars form rapidly after central stellar mass exceeds dark matter.

Tidal perturbations can induce bars in dark matter-dominated galaxies.

Secular and tidal bars differ in host galaxy structure and longevity.

Abstract

We investigate the formation of stellar bars in 307 Milky Way-mass disc galaxies in the TNG50 cosmological simulation. Most bars form rapidly in dynamically cold discs shortly after the central stellar mass exceeds that of dark matter. In these cases, bar formation is consistent with secular instabilities driven by the disc's self-gravity, which organises stellar orbits into a coherent bar structure. However, around 25 per cent of barred galaxies are dark matter dominated at the time of bar formation, $t_{bar}$, and remain so thereafter. We trace the origin of these bars to tidal perturbations induced by passing satellites or mergers using a new metric, $S_{bar}$, quantifying the tidal field acting on the galaxy. At the time of bar formation, we find a negative correlation between $S_{bar}$ and the central stellar-to-dark matter mass fraction, indicating that more dark matter-dominated discs require stronger tides to trigger bar formation. These tidally induced bars are more likely to be transient than those that form secularly, although bar properties are otherwise similar. However, the host galaxies differ: secular bars arise in relatively compact discs, while tidal bars appear in extended discs whose properties resemble those of unbarred galaxies. Tidal perturbations can therefore induce bars in galaxies otherwise stable to secular formation, highlighting the dual role of the internal galactic structure and the external environment in bar formation.