Structures induced by companions in galactic discs

TL;DR

This study uses advanced N-body simulations to explore how repeated flybys of a companion galaxy induce long-lasting structures like bars and spirals in galactic discs, revealing the importance of companion mass, disc temperature, and mode interactions.

Contribution

The paper introduces a new 3D simulation code, MAIN, and demonstrates how specific parameters influence the formation of non-axisymmetric structures in galactic discs due to flybys.

Findings

Companions with specific mass ranges induce long-lasting structures.

Higher Toomre's Q stabilizes against self-excited modes, favoring companion-induced features.

Spiral structures resemble density waves with nearly constant pattern speed.

Abstract

Using N-body simulations we study the structures induced on a galactic disc by repeated flybys of a companion in decaying eccentric orbit around the disc. Our system is composed by a stellar disc, bulge and live dark matter halo, and we study the system's dynamical response to a sequence of a companion's flybys, when we vary i) the disc's temperature (parameterized by Toomre's Q-parameter) and ii) the companion's mass and initial orbit. We use a new 3D Cartesian grid code: MAIN (Mesh-adaptive Approximate Inverse N-body solver). The main features of MAIN are reviewed, with emphasis on the use of a new Symmetric Factored Approximate Sparse Inverse (SFASI) matrix in conjunction with the multigrid method that allows the efficient solution of Poisson's equation in three space variables. We find that: i) companions need to be assigned initial masses in a rather narrow window of values in order to produce significant and more long-standing non-axisymmetric structures (bars and spirals) in the main galaxy's disc by the repeated flyby mechanism. ii) a crucial phenomenon is the antagonism between companion-excited and self-excited modes on the disc. Values of $Q >1.5$ are needed in order to allow for the growth of the companion-excited modes to prevail over the the growth of the disc's self-excited modes. iii) We give evidence that the companion-induced spiral structure is best represented by a density wave with pattern speed nearly constant in a region extending from the ILR to a radius close to, but inside, corotation.