Role of galactic bars in the formation of spiral arms: a study through orbital and escape dynamics—I

In this work, we have developed a three-dimensional gravitational model of barred galaxies, in order to study orbital and escape dynamics of the stars inside their central barred region. Our gravitational model is composed of four components: central bulge, bar, disc and dark matter halo. Furthermore, we have analysed the model for two different types of bar potentials. The study has been carried out for a Hamiltonian system, and thorough numerical investigations has been done in order to explore the regular and chaotic motions of stars. We have seen that escape mechanism has only seen near saddle points ( $$L_1$$ , $$L_2$$ and $$L_1^{'}$$ , $$L_2^{'}$$ ) of the Hamiltonian system. Orbital structures in the $$x-y$$ plane indicate that this escaping motion corresponds to the two ends of the bar. Classifications of orbits are found by calculating maximal Lyapunov exponent of the stellar trajectories corresponding to a specific initial condition vector. Poincare surface section maps are studied in both the $$x-y$$ and $$x-p_x$$ ( $$p_x$$ is the momentum along the x-direction) planes to get a complete information about the escape properties of the system in the phase space. Also, we studied in detail how the chaotic dynamics varies with the mass, length and nature of the bar. We found that under suitable physical conditions the chaos plays a pivotal role behind the formation of grand design or less prominent spiral patterns for stronger bars and ring structures for weaker bars.