http://arxiv.org/abs/1605.03379
The secular thickening of a self-gravitating stellar galactic disc embedded in a fluctuating potential is investigated. The thick WKB limit for the diffusion coefficient of the corresponding dressed Fokker-Planck equation is found using the epicyclic approximation, while assuming that only radially tightly wound transient spirals are sustained by the disc. This yields a simple quadrature, providing a clear understanding of the positions of maximum vertical orbital diffusion within the disc. This thick limit also offers 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. When applied to a tepid stable tapered disc perturbed by shot noise, this formalism predicts the formation of ridges of resonant orbits towards larger vertical actions, as found in simulations. Potential fluctuations within the disc statistically induce a vertical bending of a subset of resonant orbits, triggering the corresponding increase in vertical velocity dispersion. A simple ad hoc model of fading sequences of slowing bars explains how such a diffusion process also provides a possible mechanism allowing for the resonant thickening of galactic discs over secular timescales: the faster the fading the broader the effect.
J. Fouvry, C. Pichon and L. Monk
Thu, 12 May 16
42/59
Comments: 26 pages, 24 figures, submitted to MNRAS
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