# Mapping the stability of stellar rotating spheres via linear response theory

## TL;DR

This paper investigates the stability of rotating spherical stellar systems using linear response theory, revealing how rotation and anisotropy influence unstable modes and providing stability criteria.

## Contribution

It adapts the response matrix method to spherical rotating systems and maps stability boundaries considering anisotropy and rotation.

## Key findings

- Identification of unstable modes related to anisotropy and rotation.
- Mapping of marginal stability boundaries in parameter space.
- Comparison of theoretical predictions with N-body simulations.

## Abstract

Rotation is ubiquitous in the Universe, and recent kinematic surveys have shown that early type galaxies and globular clusters are no exception. Yet the linear response of spheroidal rotating stellar systems has seldom been studied. This paper takes a step in this direction by considering the behaviour of spherically symmetric systems with differential rotation. Specifically, the stability of several sequences of Plummer spheres is investigated, in which the total angular momentum, as well as the degree and flavour of anisotropy in the velocity space are varied. To that end, the response matrix method is customised to spherical rotating equilibria. The shapes, pattern speeds and growth rates of the systems' unstable modes are computed. Detailed comparisons to appropriate N-body measurements are also presented. The marginal stability boundary is charted in the parameter space of velocity anisotropy and rotation rate. When rotation is introduced, two sequences of growing modes are identified corresponding to radially and tangentially-biased anisotropic spheres respectively. For radially anisotropic spheres, growing modes occur on two intersecting surfaces (in the parameter space of anisotropy and rotation), which correspond to fast and slow modes, depending on the net rotation rate. Generalised, approximate stability criteria are finally presented.