Because the original theoretical motivation for nested-bar systems was to provide a mechanism for fuelling AGN Shlosman et al. (1989), the question of whether double bars actually enhance nuclear activity has been a relatively popular one.
Most recent observational studies actually suggest that inner bars play at best only a minor role in promoting nuclear activity. Martini & Pogge (1999) and Martini et al. (2001) noted that nuclear bars were not common in a small sample of local Seyfert galaxies; Erwin & Sparke (2002) compared single- and double-barred galaxies and found no significant difference in nuclear activity.
The study of Laine et al. (2002), which used larger samples of Seyfert and non-Seyfert galaxies, seems at first glance to indicate a correlation between double bars and AGN: the fraction of galaxies in their Seyfert sample with two bars is 21%, versus only 13% for the non-Seyfert galaxies. However, this is primarily a function of the higher overall bar fraction in their Seyfert galaxies. If we restrict ourselves to barred galaxies, then the fraction of (barred) Seyferts with inner bars is not significantly higher than that for the control galaxies (29% vs. 25%).
A comparison of bar frequencies for different bar sizes in the Laine et al. study yields a curious result: galaxies hosting very small bars (radii < 1 kpc), whether inner or not, are roughly as common in the Seyfert and non-Seyfert samples (Kolmogorov-Smirnov P = 0.67). But galaxies with very large bars (radii > 3 kpc) are far more likely to be Seyferts (48% of Sy galaxies, 21% of control galaxies; K-S P = 0.0051). So if bars fuel nuclei, it may only be large bars that matter.
I would like to thank the conference organizers for such an excellent and stimulating workshop, as well as for the opportunity to present this talk - and, in addition, for the chance to see the original site of the pioneering dissections by Vesalius and his successors, which help inspire part of this talk. This work was supported by Priority Programme 1177 of the Deutsche Forschungsgemeinschaft.