Dynamical Decoupling of Nested Bars: Self-Gravitating Gaseous Nuclear   Bars
  A substantial fraction of barred galaxies host additional nuclear bars which 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 which 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 Omega_s~a^{-1}, where "a" stands for the bar size. As a result, Omega_s 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_o/yr over ~10^8 yrs across the central 200 pc and has important implications for fueling the nuclear starbursts and AGN. 
Peter Englmaier (Astronomisches Institut, Basel) and Isaac Shlosman   (University of Kentucky)