BARS WITHIN BARS IN LENTICULAR AND SPIRAL GALAXIES - A STEP IN SECULAR EVOLUTION

We review observations of spiral or lenticular barred galaxies having inside their primary bar either a misaligned inner (secondary) bar or a misaligned triaxial bulge. Some CCD near-IR images are presented. Ga laxies with a nuclear ring, as well as our Galaxy, are considered as o ther likely candidates for having bars within bars. Some cases cannot be explained in terms of projection effects on two perpendicular bars. No favoured angle between the two stellar bars is found. The secondar y bar either leads or trails the primary bar, strongly indicating time -dependent systems rotating with two different pattern speeds. Offsets between a stellar bar and a gaseous bar can be explained with identic al pattern speeds under particular conditions. Models of embedded or n ested bars, produced by self-consistent 3D N-body simulations with gas and stars, are presented and the necessary and specific conditions fo r them to be formed are given. Dissipation as well as a moderate inner Lindblad resonance play a major role to induce and maintain the decou pling between the central and outer galaxy parts. The double-barred sy stems are stable over more than 5 turns of the secondary bar which is also the faster one. In some cases, a nuclear gaseous ring is formed n ear the secondary bar end. The system of embedded bars transports amou nts of gas closer to the galactic center and may be invoked as a likel y mechanism to fuel active galactic nuclei. The two bar phase is follo wed by the dissolution of the secondary bar or even the two bars (depe nding on the model) which is induced by a broad and strong inner Lindb lad resonance. The final central galaxy shape resulting from the two b ar destruction is similar to observed triaxial bulges, suggesting that some of them are relics of destroyed bars. Thus, disc galaxies can be viewed as time-dependent systems, where secular evolution, especially due to dissipative effects, is strongly transforming their morphologi es and their kinematics over less than a Hubble time. Secular evolutio n causes galaxies to change their Hubble type during their lifetimes.