Cold kinetic theory is not cold fluid theory: A collective mode in self-gravitating spheres

Abstract

Cold collisionless matter is usually described by pressureless fluid equations; this fails in three-dimensional spherical geometry. For the cold spherical case, kinetic and fluid descriptions share the same time-domain dynamics, but if we look at the spectrum, the kinetic system admits one solution the fluid does not: a discrete mode at twice the central orbital frequency, $ω_0 = 2Ω(0)$. It is specific to 3D: razor-thin disks reduce to the fluid description. Nonlinear simulations confirm it survives at finite dispersion. Around Sgr~A$^*$ it lies at $κ(r_{\rm in})$, with $r_{\rm in}$ the cusp's inner edge; if excited, stars at all radii would oscillate at this one frequency.