The orbital structure of a tidally induced bar

TL;DR

This study analyzes the orbital structures within a tidally induced galactic bar using N-body simulations, revealing a predominance of boxy orbits and diverse frequency ratios, which enhances understanding of galaxy evolution.

Contribution

It provides a detailed classification of orbits in a tidally induced bar, highlighting the dominance of boxy shapes and the presence of near-periodic orbits with specific frequency ratios.

Findings

Over 80% of orbits are boxy in shape.

Only 8% of orbits are classical x1 type.

Orbits grow more elongated with distance from the center.

Abstract

Orbits are the key building blocks of any density distribution and their study helps us understand the kinematical structure and the evolution of galaxies. Here we investigate orbits in a tidally induced bar of a dwarf galaxy, using an $N$-body simulation of an initially disky dwarf galaxy orbiting a Milky Way-like host. After the first pericenter passage, a tidally induced bar forms in the stellar component of the dwarf. The bar evolution is different than in isolated galaxies and our analysis focuses on the period before it buckles. We study the orbits in terms of their dominant frequencies, which we calculate in a Cartesian coordinate frame rotating with the bar. Apart from the well-known x$_1$ orbits we find many other types, mostly with boxy shapes of various degree of elongation. Some of them are also near-periodic, admitting frequency ratios of 4/3, 3/2 and 5/3. The box orbits have various degrees of vertical thickness but only a relatively small fraction of those have banana (i.e. smile/frown) or infinity-symbol shapes in the edge-on view. In the very center we also find orbits known from the potential of triaxial ellipsoids. The elongation of the orbits grows with distance from the center of the bar in agreement with the variation of the shape of the density distribution. Our classification of orbits leads to the conclusion that more than $80 \%$ of them have boxy shapes, while only $8 \%$ have shapes of classical x$_1$ orbits.