Bar‐Halo Friction in Galaxies. II. Metastability

It is well-established that strong bars rotating in dense halos generallyslow down as they lose angular momentum to the halo through dynamical friction.Angular momentum exchanges between the bar and halo particles take place atresonances. While some particles gain and others lose, friction arises whenthere is an excess of gainers over losers. This imbalance results from thegenerally decreasing numbers of particles with increasing angular momentum, andfriction can therefore be avoided if there is no gradient in the density ofparticles across the major resonances. Here we show that anomalously weakfriction can occur for this reason if the pattern speed of the bar fluctuatesupwards. After such an event, the density of resonant halo particles has alocal inflexion created by the earlier exchanges, and bar slowdown can bedelayed for a long period; we describe this as a metastable state. We show thatthis behavior in purely collisionless N-body simulations is far more likely tooccur in methods with adaptive resolution. We also show that the phenomenoncould arise in nature, since bar-driven gas inflow could easily raise the barpattern speed enough to reach the metastable state. Finally, we demonstratethat mild external, or internal, perturbations quickly restore the usualfrictional drag, and it is unlikely therefore that a strong bar in a galaxyhaving a dense halo could rotate for a long period without friction.Comment: 13 pages, 11 figures, to appear in Ap