Double X/Peanut Structures in Barred Galaxies -- Insights from an
  $N$--body Simulation
  Boxy, peanut- or X-shaped "bulges" are observed in a large fraction of barred
galaxies viewed in, or close to, edge-on projection, as well as in the Milky
Way. They are the product of dynamical instabilities occurring in stellar bars,
which cause the latter to buckle and thicken vertically. Recent studies have
found nearby galaxies that harbour two such features arising at different
radial scales, in a nested configuration. In this paper we explore the
formation of such double peanuts, using a collisionless N-body simulation of a
pure disc evolving in isolation within a live dark matter halo, which we
analyse in a completely analogous way to observations of real galaxies. In the
simulation we find a stable double configuration consisting of two X/peanut
structures associated to the same galactic bar - rotating with the same pattern
speed - but with different morphology, formation time, and evolution. The
inner, conventional peanut-shaped structure forms early via the buckling of the
bar, and experiences little evolution once it stabilises. This feature is
consistent in terms of size, strength and morphology, with peanut structures
observed in nearby galaxies. The outer structure, however, displays a strong X,
or "bow-tie", morphology. It forms just after the inner peanut, and gradually
extends in time (within 1 to 1.5 Gyr) to almost the end of the bar, a radial
scale where ansae occur. We conclude that, although both structures form, and
are dynamically coupled to, the same bar, they are supported by inherently
different mechanisms.
