Dynamical Decoupling of Nested Bars: Self-gravitating Gaseous Nuclear Bars

Abstract

A substantial fraction of barred galaxies host additional nuclear bars that tumble with pattern speeds exceeding those of the large-scale (primary) stellar bars. We have investigated the mechanism of formation and dynamical decoupling in such nested bars that include gaseous (secondary) nuclear bars within the full-size galactic disks, hosting a double inner Lindblad resonance. Becoming increasingly massive and self-gravitating, the nuclear bars lose internal (circulation) angular momentum to the primary bars and increase their strength. Developing chaos within these bars triggers a rapid gas collapse—bar contraction. During this time period, the secondary bar pattern speed Ωs ~ a-1, where a stands for the bar size. As a result, Ωs increases dramatically until a new equilibrium is reached (if at all), while the gas specific angular momentum decreases—demonstrating the dynamical decoupling of nested bars. Viscosity, and therefore the gas presence, appears to be a necessary condition for the prograde decoupling of nested bars. This process maintains an inflow rate of ~1 M☉ yr-1 over ~108 yr across the central 200 pc and has important implications for fueling the nuclear starbursts and active galactic nuclei.