Radial-orbit instability in modified Newtonian dynamics

TL;DR

This study investigates the stability of radially anisotropic spherical stellar systems in modified Newtonian dynamics (MOND) through numerical simulations, revealing that MOND systems are more prone to radial-orbit instability than dark matter models.

Contribution

The paper provides the first detailed numerical comparison of radial-orbit instability in MOND versus Newtonian models with and without dark matter.

Findings

MOND models require larger minimum anisotropy radius for stability.

Maximum stability parameter in MOND is lower than in dark matter models.

MOND systems are more prone to radial-orbit instability than dark matter systems.

Abstract

The stability of radially anisotropic spherical stellar systems in modified Newtonian dynamics (MOND) is explored by means of numerical simulations performed with the N-body code N-MODY. We find that Osipkov-Merritt MOND models require for stability larger minimum anisotropy radius than equivalent Newtonian systems (ENSs) with dark matter, and also than purely baryonic Newtonian models with the same density profile. The maximum value for stability of the Fridman-Polyachenko-Shukhman parameter in MOND models is lower than in ENSs, but higher than in Newtonian models with no dark matter. We conclude that MOND systems are substantially more prone to radial-orbit instability than ENSs with dark matter, while they are able to support a larger amount of kinetic energy stored in radial orbits than purely baryonic Newtonian systems. An explanation of these results is attempted, and their relevance to the MOND interpretation of the observed kinematics of globular clusters, dwarf spheroidal and elliptical galaxies is briefly discussed.