4.5. Decoupling of a nuclear disk
The bar torques drive progressively more mass towards the center. This matter, gaseous at the beginning, forms stars, and gradually contributes to the formation of the bulge, since stars are elevated above the disk plane, through vertical resonances with the bar (e.g. Combes et al. 1990, Raha et al. 1991). When the mass accumulation is large enough, then the precessing rate - / 2 curve is increasing strongly while the radius decreases, and this implies the formation of two inner Lindblad resonances. In between the two ILRs, the periodic orbits are perpendicular to the bar (x2 orbits), and the bar loses its main supporters. The weakening of the primary bar, and the fact that the frequencies of the matter are considerably different now between the inner and outer disk, forces the decoupling of a nuclear disk, or nuclear bar from the large-scale bar (primary bar).
Nuclear disks are frequently observed, at many wavelengths: optical with the HST (e.g. Barth et al 1995, or fig 10) or in CO molecules with millimeter interferometers (Ishizuki et al 1990). A recent survey in the Virgo cluster (Rubin et al 1997) reveals that about 20% of the 80 spirals observed possess a decoupled nuclear disk. In nearly edge-on systems, these nuclear disks are conspicuous through large velocity gradients, like in the Milky-Way (Dame et al 1987) or NGC 891 (Garcia-Burillo & Guélin 1995).
Figure 10. Photographs of the barred galaxy NGC 4314. (top) A general view of the bar and the spiral arms (photo from McDonald Observatory, Texas). (bottom) A zoom on the central parts (corresp. to the square in the first picture), with the Hubble Space Telescope. A nuclear ring can be discerned, within which a second independent spiral structure has developed (from Benedict et al. (1996).)