The properties of cuspy triaxial systems formed due to the radial-orbit instability

TL;DR

This study demonstrates that radial-orbit instability naturally produces stable, triaxial equilibrium models with invariant cuspy density profiles, useful for modeling elliptical galaxies and bulges.

Contribution

It introduces a new technique for constructing equilibrium N-body models by exploiting radial-orbit instability to generate stable, cuspy, triaxial systems from spherical progenitors.

Findings

Density profiles remain unchanged during shape transformation.

Cusps are invariant features of the final triaxial systems.

Final models are anisotropic and resemble elliptical galaxy structures.

Abstract

Context. We have investigated the structure and kinematics of triaxial final models formed due to radial-orbit instability from a set of equilibrium anisotropic spherical systems of the Osipkov-Merritt type. Aims. We show that the instability is a natural way to build nonspherical systems and thus can be considered as a new technique for constructing equilibrium $N$-body models. Methods. The dynamical evolution of the models was followed numerically by means of the Dehnen's public code gyrfacON. Results. We found that the shape of the density profiles for various initial spherical models didn't change despite of the systems were drastically rearranged into triaxial configurations and had got the new scale lengths. The cusp was found to be an invariant feature of such a rearranging. Starting from a certain unstable spherical $\gamma$-model we obtain a triaxial system with the same cusp. The end-products are anisotropic at large radii and have anisotropy profiles of the Osipkov-Merrit type. The size of the isotropic core once rescaled corresponds to the value of marginally stable $\gamma$-progenitors and is almost independent of the unstable starting point. Conclusions. We conclude that the end-products reach a new steady state and have quite predictable properties. They can be used as equilibrium models to describe the elliptical galaxies and bulges. They also can be incorporated in $N$-body simulations deal with multicomponent systems.