It is well established now that bars are very often found in disc galaxies. One usually finds a bar fraction of ~ 2/3 [32], considering both prominent and weak bars, unless one is looking for bars in images observed at too short wavelengths [86], as the stellar content of bars is usually dominated by old, red stars. In fact, most bars fade away in ultra-violet images [44], although some bars can still be recognised (cf. NGC 1097). Even though the remaining ~ 1/3 of disc galaxies do not seem to harbour identifiable bars, they might still have less prominent non-axisymmetric distortions.
In addition, bars are seen in galaxies with a wide range of bulge-to-total ratio and mass, i.e. from lenticulars to irregulars. Thus, secular evolution processes induced by bars occur not only in disc-dominated galaxies with inconspicuous bulges. They also happen in bulge-dominated galaxies, which suggests the coexistence of classical, merger-built bulges, with bulges built from disc dynamical instabilities [51]. But secular evolution can also happen without bars. Oval distortions in discs can also induce a substantial exchange of angular momentum from the inner to the outer parts of galaxies [51].
Finally, a recent development suggests that the total amount of mass
within stars that reside in bars at z
0 is similar to that
kept in stars belonging to classical bulges. A similar amount is
confined to elliptical galaxies. Approximately 15% of the total mass in
stars at z
0 is located in bars.
Classical bulges and
elliptical galaxies contain each a comparable fraction. This means that,
as far as the stellar mass budget in the local universe is concerned,
bars are as relevant as classical bulges and elliptical galaxies. The
other ~ 1/2 of the stellar mass content at z
0 is in
galaxy discs
[36,
24].