Double bars in disk galaxies: Dynamical decoupling of non-self-gravitating gaseous bars

We find that nuclear rings in barred galaxies can be subject to a new type of non-self-gravitational dynamical instability. The instability leads to the formation of gaseous molecular bars with pattern speeds that are substantially slower than speeds of the primary stellar bars. This spectacular decoupling of nuclear bars from the underlying gravitational potential is triggered but is not driven by the gas viscosity. We find that low-viscosity systems can spend a substantial period of time in a fully decoupled state, with the nuclear bar slowly tumbling in the gravitational field of the primary bar. Higher viscosity systems form nuclear bars that librate about the primary bar. The shape of a nuclear bar, i.e., its eccentricity, correlates strongly with the angle between the bars. We also find that such decoupling, partial or full, most probably will be associated with bursts of star formation and with gas inflow across the inner (Lindblad) resonance zone toward smaller radii.