Multiple pattern speeds in a long peanut-shaped bar in a simulated
  galaxy
  A significant fraction of barred spiral galaxies exhibits peanut/X-shaped
structures in their central regions. Bars are known to rotate with a single
pattern speed, and they eventually slow down over time due to the dynamical
friction with the surrounding dark matter halo. However, the nature of the
decay in pattern speed values and whether all peanut bars rotate with a single
pattern speed remain to be investigated. Using N-body simulation of a
collisionless stellar disc, we study the case of a long bar with a
three-dimensional peanut structure prominent in both edge-on and face-on
projections. We show that such a bar possesses three distinct peaks in the m=2
Fourier component. Using the Tremaine-Weinberg method, we measure the pattern
speeds and demonstrate that the three regions associated with the three peaks
rotate with different pattern speeds. The inner region, which is the core of
the peanut, rotates slower than the outer regions. In addition, the pattern
speed of the inner bar also decays faster than the outer bar with a decay
timescale of 4.5 Gyr for the inner part and ~12.5 Gyr for the outer parts. This
is manifested as a systematic offset in density and velocity dispersion maps
between the inner and outer regions of the long peanut bar. We discuss the
importance of our findings in the context of bar dynamics.
