Bar-Halo Friction in Galaxies II: Metastability

TL;DR

This paper explores how fluctuations in bar pattern speed can create metastable states in galaxy simulations, temporarily reducing dynamical friction and affecting bar slowdown in dense halos.

Contribution

It introduces the concept of metastability caused by pattern speed fluctuations, explaining delayed bar slowdown in collisionless galaxy models.

Findings

Weak friction occurs after upward pattern speed fluctuations.

Metastable states can last long in adaptive resolution simulations.

External perturbations restore typical frictional behavior.

Abstract

It is well-established that strong bars rotating in dense halos generally slow down as they lose angular momentum to the halo through dynamical friction. Angular momentum exchanges between the bar and halo particles take place at resonances. While some particles gain and others lose, friction arises when there is an excess of gainers over losers. This imbalance results from the generally decreasing numbers of particles with increasing angular momentum, and friction can therefore be avoided if there is no gradient in the density of particles across the major resonances. Here we show that anomalously weak friction can occur for this reason if the pattern speed of the bar fluctuates upwards. After such an event, the density of resonant halo particles has a local inflexion created by the earlier exchanges, and bar slowdown can be delayed for a long period; we describe this as a metastable state. We show that this behavior in purely collisionless N-body simulations is far more likely to occur in methods with adaptive resolution. We also show that the phenomenon could arise in nature, since bar-driven gas inflow could easily raise the bar pattern speed enough to reach the metastable state. Finally, we demonstrate that mild external, or internal, perturbations quickly restore the usual frictional drag, and it is unlikely therefore that a strong bar in a galaxy having a dense halo could rotate for a long period without friction.