Abstract
We analyse 307 Milky Way-mass disc galaxies in the TNG50 cosmological simulation to study the formation and evolution of stellar bars through secular processes and tidal interactions. About 90 per cent of these galaxies form at least one bar during their cosmic evolution. Most bars form rapidly in dynamically cold discs shortly after the central stellar mass exceeds that of dark matter (inside the stellar half-mass radius). In these cases, bar formation is consistent with secular instabilities driven by the disc's self-gravity, which organizes stellar orbits into a coherent bar structure. However, about 24 per cent of barred galaxies are dark matter-dominated at the time of bar formation. We trace the origin of these bars to tidal perturbations from passing or accreting satellites and streams, and introduce a new metric, [Formula: see text], to quantify the total external tidal field acting on each galaxy. We find that [Formula: see text] correlates negatively with the central stellar-to-dark matter mass ratio at the time of bar formation: the more dark-matter-dominated a disc, the stronger the tidal perturbation required to trigger a bar. Bars that form in dark matter-dominated discs under tidal perturbations are typically transient - disappearing in a few Gyr - unlike their counterparts which form in stellar-dominated discs. After a few orbital times, the properties of all bars are broadly similar, though their host galaxies differ: secular bars arise in thin, compact discs, whereas tidally induced bars can form in thicker, more extended discs whose properties closely resemble those of unbarred galaxies.