D. Friedli et L. Martinet, BARS WITHIN BARS IN LENTICULAR AND SPIRAL GALAXIES - A STEP IN SECULAR EVOLUTION, Astronomy and astrophysics, 277(1), 1993, pp. 27-41
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.