Multiple pattern speeds in a long peanut-shaped bar in a simulated galaxy

TL;DR

This study uses N-body simulations to reveal that long peanut-shaped bars in galaxies can have multiple pattern speeds, with the inner region rotating slower and decaying faster than the outer parts, challenging the notion of a single pattern speed.

Contribution

It demonstrates that long peanut-shaped bars can have three distinct pattern speeds and decay rates, providing new insights into complex bar dynamics in galaxies.

Findings

Long peanut-shaped bars exhibit three peaks in the m=2 Fourier component.

Different regions of the bar rotate with distinct pattern speeds.

Inner bar regions decay faster than outer regions, with decay times of 4.5 Gyr and 12.5 Gyr respectively.

Abstract

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.