Time-dependent Pattern Speeds in Barred Galaxies

TL;DR

This study uses high-quality N-body simulations to analyze how the pattern speeds of double bars in galaxies evolve over time, revealing complex interactions and oscillations that challenge traditional assumptions about stellar actions.

Contribution

It provides a detailed analysis of the time-dependent pattern speeds in double barred galaxies using multiple measurement methods, highlighting the influence of bar interactions and spiral structures.

Findings

Inner bar's size, strength, and pattern speed vary with bar interactions.

Outer bar's properties are affected by nearby spiral structures.

Pattern speed ratio between bars increases over time, with high-frequency fluctuations.

Abstract

Based on a high quality $N$-body simulation of a double bar galaxy model, we investigate the evolution of the bar properties, including their size, strength and instantaneous pattern speed derived by using three distinct methods: the Fourier, Jacobi integral, and moment of inertia methods. The interaction of the two bars, which rotate at distinct speeds, primarily affects the size, strength and pattern speed of the inner bar. When the two bars are perpendicular to each other, the size and the pattern speed of the inner bar decrease and its strength increases. The emergence of a strong Fourier $m=1$ mode increases the oscillation amplitude of the size, strength and pattern speed of the inner bar. On the other hand, the characteristics of the outer bar are substantially influenced by its adjacent spiral structure. When the spiral structure disappears, the size of the outer bar increases and its strength and pattern speed decrease. Consequently, the ratio of the pattern speed of the outer bar with respect to the inner bar is not constant and increases with time. Overall, the double bar and disk system displays substantial high frequency semi-chaotic fluctuations of the pattern strengths and speeds both in space and time, superposed on the slow secular evolution, which invalidates the assumption that the actions of individual stars should be well conserved in barred galaxies, such as the Milky Way.