A mechanism of bar formation in disk galaxies: synchronization of apsidal precession

Abstract

Abstract We discuss the mechanism(s) of bar formation in isolated and tidally interacting disk galaxies using the results of idealized collisionless Nbody simulations of the galaxies. In order to better understand the mechanism, we investigate orbital eccentricities (e), epochs of apocenter passages (ta), azimuthal angles at ta (ϕa), precession rates (Ωpre), for individual stars, as well as bar strengths represented by relative m = 2 Fourier amplitude (A2) and bar pattern speeds (Ωbar). The main results are as follows. A significant fraction of stars with initially different ϕa and Ωpre in an isolated disk galaxy can have similar values within several dynamical timescales. This synchronization of ϕa and Ωpre, which is referred to as apsidal precession synchronization (‘APS’) in the present study, is caused by the enhanced strength of the tangential component of gravitational force. A weak seed bar (A2 < 0.1) is first formed through APS in local regions of a disk, then the bar grows due to APS. In the bar growth phase (0.1 < A2 < 0.4), APS can proceed more efficiently due to stronger tangential force from the bar so that it can enhance the bar strength further. This positive feedback loop in APS is the key physical mechanism of bar growth in isolated stellar disks. Bar formation can be severely suppressed in disks with lower disk mass fractions and/or higher Q parameters due to much less efficient APS. APS proceeds more rapidly and more efficiently due to strong tidal perturbation in the formation of tidal bars compared to spontaneous bar formation.