4.6. Bars within bars

Secondary bars form through decoupling (Friedli & Martinet 1993, Combes 1994). The second bars rotate with a much faster angular velocity, and are observed with a random angle from the primary bar (see fig 11). To avoid chaos, the two bars have a resonance in common. It is frequent that the ILR of the primary coincides with the corotation of the secondary. Multiply periodic particle orbits have been identified in such time-varying potentials (Maciejewski & Sparke 1997). It is possible that the two bars exchange energy with each other, through non-linear coupling; then m = 4 and m = 0 modes are also expected, and these have been seen in simulations (Tagger et al. 1987, Masset & Tagger 1997). Even then, the life-time of the ensemble is rather short, a few rotations. But the nuclear bars could help to prolonge the action of the primary bar towards the nucleus (as first proposed by Shlosman et al. 1989).

Figure 11. Examples of nuclear bars seen in the NIR bands: left: NGC 1433 in H; right: NGC 2217 in H; the field of view of both images is 2 x 2 arcmin, from Jungwiert et al (1997).