Resonant thickening of self-gravitating discs: imposed or self-induced orbital diffusion in the tightly wound limit

Abstract

The secular thickening of a self-gravitating stellar galactic disc is investigated using the dressed collisionless Fokker-Planck equation and the inhomogeneous multicomponent Balescu-Lenard equation. The thick WKB limits for the diffusion fluxes are found using the epicyclic approximation, while assuming that only radially tightly wound transient spirals are sustained by the disc. This yields simple quadratures for the drift and diffusion coefficients, providing a clear understanding of the positions of maximum vertical orbital diffusion within the disc, induced by fluctuations either external or due to the finite number of particles. These thick limits also offer a consistent derivation of a thick disc Toomre parameter, which is shown to be exponentially boosted by the ratio of the vertical to radial scale heights. Dressed potential fluctuations within the disc statistically induce a vertical bending of a subset of resonant orbits, triggering the corresponding increase in vertical velocity dispersion. When applied to a tepid stable tapered disc perturbed by shot noise, these two frameworks reproduce qualitatively the formation of ridges of resonant orbits towards larger vertical actions, as found in direct numerical simulations, but overestimates the time-scale involved in their appearance. Swing amplification is likely needed to resolve this discrepancy, as demonstrated in the case of razor-thin discs. Other sources of thickening are also investigated, such as fading sequences of slowing bars, or the joint evolution of a population of giant molecular clouds within the disc.

31 pages, 19 figures

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Cited by in corpus (13)

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• Exploring the origin of thick disks using the NewHorizon and Galactica simulations
• Relaxation in self-gravitating systems
• Isotropic-Nematic Phase Transitions in Gravitational Systems II: Higher Order Multipoles
• The structure of the Milky Way based on unWISE 3.4$\mu$m integrated photometry
• Kinetic theory of one-dimensional homogeneous long-range interacting systems with an arbitrary potential of interaction
• Long-term relaxation of $1D$ self-gravitating systems
• The secular evolution of discrete quasi-Keplerian systems. II. Application to a multi-mass axisymmetric disc around a supermassive black hole
• On the long-term evolution of razor-thin galactic discs: Balescu-Lenard prediction and perspectives
• Resonant Effects of a Bar on the Galactic Disk Kinematics Perpendicular to Its Plane
• Linear response of rotating and flattened stellar clusters: the oblate Kuzmin-Kutuzov Stäckel family
• Emergence of the Galaxy Morphology-Star Formation Activity-Clustercentric Radius Relations in Galaxy Clusters