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

TL;DR

This paper reveals that synchronization of stellar orbital precession, driven by gravitational forces, is a key mechanism behind bar formation in disk galaxies, both isolated and tidally interacting.

Contribution

It introduces the concept of apsidal precession synchronization (APS) as a fundamental process in bar formation, supported by N-body simulation analysis.

Findings

APS causes initial weak bar formation in disks.

Bar growth is enhanced by positive feedback in APS.

Tidal interactions accelerate APS and bar formation.

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 (t_a), azimuthal angles at t_a (varphi_a), precession rates (Omega_pre), for individual stars, as well as bar strengths represented by relative m=2 Fourier amplitude (A_2) and bar pattern speeds (Omega_bar). The main results are as follows. A significant fraction of stars with initially different varphi_a and Omega_pre in an isolated disk galaxy can have similar values within several dynamical timescales. This synchronization of varphi_a and Omega_pre, 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 (A_2<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<A_2<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.