Instability of counter-rotating stellar disks

TL;DR

This study uses N-body simulations to analyze the dynamics and instabilities caused by counter-rotating stellar disks, revealing complex interactions and density wave formations relevant to galaxy evolution.

Contribution

It provides a detailed simulation-based analysis of counter-rotating disk interactions, highlighting the role of two-stream instability and resulting density structures.

Findings

Excitation of multi-armed spiral density waves in the main disk

Formation of high azimuthal mode density distributions in counter-rotating flow

Scattering of material to large radii due to instabilities

Abstract

We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. In order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in which the counter-rotating flow is in an annular region immediately exterior the main portion of the astrophysical disk. Such counter-rotating flows are seen in systems such as NGC 4826 (known as the "Evil Eye Galaxy"). Interaction between the rotating and counter-rotating components is due to two-stream instability in the boundary region. A multi-armed spiral density wave is excited in the astrophysical disk and a density distribution with high azimuthal mode number is excited in the counter-rotating flow. Density fluctuations in the counter-rotating flow aggregate into larger clumps and some of the material in the counter-rotating flow is scattered to large radii. Accretion flow processes such as this are increasingly seen to be of importance in the evolution of multi-component galactic disks.