Theoretically and numerically, bars are expected to concentrate gas in the central regions of spiral galaxies because the torqued and shocked gas within the bar loses angular momentum which allows the gas to move further in (e.g., Schwarz 1984; Combes & Gerin 1985; Noguchi 1988; Shlosman, Frank & Begelman 1989; Knapen et al. 1995a). The dynamics of bars and their influence on the circumnuclear regions has most recently been reviewed by Kormendy & Kennicutt (2004), and previously by, e.g., Sellwood & Wilkinson (1993) and Shlosman (2001). The general theoretical and numerical formalism of bars is now well understood, and different aspects of it are continuously being confirmed by observations. For instance, we recently investigated the well-known numerical result that stronger bars will lead to straight dust lanes along the leading edges of the bar, whereas the dust lanes will be more curved in weak bars (Athanassoula 1992). Using a small number of barred galaxies for which we had adequate data, we could indeed confirm observationally that there is an anti-correlation between the amount of curvature of the dust lanes and the gravitational bar torque, or bar strength (Knapen, Pérez-Ramírez & Laine 2002; see Fig. 1). In another study (Zurita et al. 2004), we used H Fabry-Pérot data of the strongly barred galaxy NGC 1530 to show in a graphic, two-dimensional way that indeed, as predicted by theory, large velocity gradients are found at the position of the dust lanes. Within those lanes, directly tracing enhanced concentrations of dust and thus gas, but indirectly tracing the location of shocks in the gas, the large velocity gradient prohibits massive star formation, which we observe to be located just outside the regions of largest shear or velocity gradient (Zurita et al. 2004; see Regan, Vogel & Teuben 1997 for an H Fabry-Pérot map at lower resolution which nevertheless indicates the shocks in the velocity field).
Figure 1. Gravitational bar torque Qg, an indicator of bar strength, as a function of the curvature of the dust lanes in a sample of 9 barred galaxies. Small values of indicate straight dust lanes, which are seen to occur in strong bars, thus confirming theoretical and numerical predictions. Data from Knapen, Pérez-Ramírez & Laine (2002).
Considering now specifically the theoretical and numerical view that bars can instigate radial inflow of gas, and thus lead to gas accumulation in the central regions of barred galaxies, several pieces of observational evidence to fit this picture have been forthcoming in recent years, both from observations of gas tracers in barred and non-barred galaxies (e.g., Sakamoto et al. 1999; Jogee, Scoville, & Kenney 2004; Sheth et al. 2004), and from other, less direct, measures of the gas concentration (e.g., Maiolino, Risaliti, & Salvati 1999; Alonso-Herrero & Knapen 2001). These results have been reviewed in somewhat more detail by Knapen (2004a), and we limit ourselves here to the conclusion that there is increasing observational support for the theoretical suggestion that bars lead to gas accumulation in the central regions of galaxies.
One must keep in mind that in all cases the observed correlation between the presence of a bar and increased central gas concentration is statistical, and not very strong, that there is a large overlap region in which the properties of barred and non-barred galaxies are very similar indeed (for instance, in the study by Sakamoto et al. 1999 just over half of the 19 sample galaxies are in the overlap range of gas concentration parameter tcon, which is inhabited by both barred and non-barred galaxies), and that in the CO studies the X factor which gives the transformation of CO luminosity to mass is assumed to have the same value in the circumnuclear regions and in the disk. One can also question whether the statistical gas accumulation by bars is in fact related to the occurrence of nuclear activity of the non-stellar or stellar variety, and a careful consideration of both spatial- and timescales must be made to connect gravitationally driven inflow to fuelling of the starburst and/or the AGN. Finally, there is as yet no convincing direct observational evidence of inflow in a barred galaxy, mainly because the inflow rates are so low that they may be unobservable in practive (see above), and because most of the gas in bars moves around the bar, and will thus move inward during a part of its orbit, but then move outward again on a subsequent part (see discussion in Knapen 2001).
In the remainder of this Section, we will review the question of whether there is observational evidence that bars are related to the occurrence of nuclear activity.