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 ofnon-self-gravitational dynamical instability. The instability leads to theformation of gaseous molecular bars with pattern speeds which are substantiallyslower than speeds of the primary stellar bars. This spectacular decoupling ofnuclear bars from the underlying gravitational potential is triggered but isnot driven by the gas viscosity. We find that low-viscosity systems can spend asubstantial period of time in a fully decoupled state, with the nuclear barslowly tumbling in the gravitational field of the primary bar. Higher viscositysystems form nuclear bars which librate about the primary bar. The shape of anuclear bar, i.e., its eccentricity, correlates strongly with the angle betweenthe bars. We also find that such decoupling, partial or full, most probablywill be associated with bursts of star formation and with gas inflow across theinner (Lindblad) resonance zone towards smaller radii.Comment: 10 pages, Latex plus 8 figures, ApJ double column macros, mpeg simulations available from the on-line edition of the Astrophysical Journal, or from authors. to be published in Astrophysical Journal, Vol. 551, April 20, 200