11. DYNAMICS OF BARS
About two-thirds of spiral galaxies possess a non-axisymmetric distortion or a bar in their stellar component, although only one-third possess a really strong bar, of SB type (e.g. de Vaucouleurs 1963). Red or near-infrared photometry has revealed many bars and oval distortions in the old stellar component that were not visible on a blue photograph of the same galaxy, because of dust and star-formation regions (Zaritsky & Lo 1986; Rix & Rieke 1993). A bar can be detected also by the cold gas component, which is a good tracer of faint perturbations in the potential (e.g. CO observations of IC 342, Ishizuki et al. 1990; NGC 6946, Ball et al. 1985; see also Turner 1996). It can therefore be concluded that a bar exists in the great majority of galaxies, and is not a peculiar structure, as was considered before the 1970's. Our own Galaxy appears barred from its kinematics and elliptical streamlines (e.g. Peters 1975; Mulder & Liem 1986), and also from its boxy and asymmetric near-infrared contours (Blitz & Spergel 1991), and its micro-lensing efficiency (Paczynski et al. 1994). Our nearby companions are also barred (M31, Large and Small Magellanic Clouds, etc.).
Observed and dynamical properties of bars have recently been nicely reviewed by Sellwood & Wilkinson (1993). As far as rings are concerned, it is interesting here to note that barred galaxies might be the only objects where a long-lived, quasi-stationary, normal mode can be recognized. Bars are essentially composed of an old population, and the spiral waves in a barred galaxy are strongly influenced (maybe driven?) by the bar. In strong bars, the spiral arms appear always in the continuation of the bar, suggesting that they rotate with the same pattern speed. The presence of a grand-design spiral is about twice as frequent in barred galaxies than in nonbarred ones, as determined by Elmegreen & Elmegreen (1983). While nonbarred galaxies can be multi-armed or stochastic, most barred galaxies possess a two-armed regular density wave. Barred galaxies are therefore ideal for studying resonance phenomena.