The radial orbit instability in collisionless N-body simulations

Eric I. Barnes, Paul A. Lanzel, Liliya L.R. Williams

Research output: Contribution to journalArticlepeer-review

29 Scopus citations


Using a suite of self-gravitating, collisionless N-body models, we systematically explore a parameter space relevant to the onset and behavior of the radial orbit instability (ROI), whose strength is measured by the systemic axis ratios of the models. We show that a combination of two initial conditions, namely the velocity anisotropy and the virial ratio, determines whether a system will undergo ROI and exactly how triaxial the system will become. A third initial condition, the radial shape of the density profile, plays a smaller, but noticeable role. Regarding the dynamical development of the ROI, the instability (1) begins after systems collapse to their most compact configuration and (2) evolves fastest when a majority of the particles have radially anisotropic orbits, while there is a lack of centrally concentrated isotropic orbits. We argue that this is further evidence that self-reinforcing torques are the key to the onset of the ROI. Our findings support the idea that a separate orbit instability plays a role in halting the ROI.

Original languageEnglish (US)
Pages (from-to)372-384
Number of pages13
JournalAstrophysical Journal
Issue number1
StatePublished - 2009


  • Galaxies: Kinematics and dynamics
  • Galaxies: Structure

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