Parametric resonance scenario for peanut bulge formation in rotating galactic stellar disks

The coupling is established analytically between the natural oscillations of stars normal to the mean horizontal plane and periodic changes in the excitation force in a rotationally flattened galactic disk using a linear perturbation framework. The nonaxisymmetric gravitational potential of a density-wave bar may cause these changes in this ‘pumping’ force. The excitation of out-of-plane oscillating motion with an amplitude exponentially increasing with time via parametric resonance interaction between stars and a bar is considered for the formation of the boxy/peanut-shaped bulges (an X-structure of stars crossing at the center of the system) often observed in disk galaxies, even in isolation, as well as in the Galaxy. A scenario involving this resonant interaction accounts for both the vertically short 1 ≲ 1 kpc boxy/peanut-shaped features and a ~ 1 ∼ 1 Gyr time delay between when the bars form and vertically thicken.