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

TL;DR

This study develops a 3D gravitational model of barred galaxies to analyze star orbital and escape dynamics, revealing how chaos influences spiral arm and ring formation depending on bar strength.

Contribution

Introduces a detailed 3D gravitational model with two bar potentials to explore orbital chaos and escape mechanisms in barred galaxies.

Findings

Escape occurs near saddle points of the Hamiltonian system.

Chaotic dynamics influence spiral and ring structures based on bar properties.

Orbital classifications are achieved via Lyapunov exponents and Poincaré maps.

Abstract

In the present 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 nucleus, 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 studies have been done in order to categorize regular and chaotic motions of stars. We have seen that escape mechanism has only seen near saddle points ($L_2$, $L_4$ and $L_2^{'}$, $L_4^{'}$) of the Hamiltonian system. Orbital structures in $x$ - $y$ 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. Poincar\'e surface section maps are studied in both $x$ - $y$ and $x$ - $p_x$ ($p_x$ is the momentum along $x$ - direction) plane to get a complete view of the escape properties of the system in the phase space. Also we studied in detail how the chaotic dynamics varies with 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 poor spiral pattern for stronger bars and ring structures for weaker bars.