The Dependency of Bar Formation Timescale on Disk Mass Fraction, Toomre $Q$, and Scale Height

Abstract

Bars are one of the most prominent galactic structures. The classical swing-amplification theory can qualitatively describe the spontaneous bar instability of stellar disks. Still, it cannot quantify the bar formation process or explain why some disk galaxies do not have a bar. Recent studies found that the bar formation timescale depends exponentially on the disk mass fraction of the host galaxy (dubbed as "Fujii relation"), but they only explored a limited parameter space, where the physical effects of Toomre $Q$ (local disk stability parameter) and disk scale height of the host galaxies are not fully explored. In this work, we check the robustness of the Fujii relation in a higher-dimensional parameter space of disk mass fraction, Toomre $Q$, and scale height. We find that the Fujii relation holds for disk galaxies with physically reasonable Toomre $Q$ and scale height. Furthermore, the bar formation timescale also approximately linearly depends on both Toomre $Q$ and scale height, with a more prolonged bar formation in a hotter or thicker disk. We propose an empirical relation to combine the dependency of the bar formation timescale on the three parameters. Based on the empirical relation and recent observations, we estimate that the bar formation timescale in pure stellar disks ranges from $0.20_{-0.06}^{+0.09}~\mathrm{Gyr}$ to $12.20_{-2.80}^{+3.37}~\mathrm{Gyr}$ or even significantly beyond the Hubble timescale in some extreme cases.