12.4. Fueling the Nucleus

Gravity torques from the bar can drive very efficiently the gas towards the center, and it has been proposed that non-axisymmetric potentials are the main agent to bring gas towards active nuclei, to fuel their strong energetic activity (Simkin et al. 1980). However, as emphasized by Combes & Gerin (1985), the gas is in general efficiently driven to the inner Lindblad resonance, where it accumulates, but not to the very center. We can try to understand schematically, in terms of orbits, the sense of winding of gaseous spiral arms in the center, and consequently the sense of gravity torques.

Figure 78. a) Periodic orbits in a cos 2 potential. Their orientation rotates by /2 at each resonance. b) The gas tends to follow these orbits, but is forced to precess more rapidly while losing energy and angular momentum.

In a cos 2 bar potential, the main families of periodic orbits are aligned or anti-aligned to the bar, according to the position with respect to Lindblad resonances (see Figure 78). Due to cloud collisions, the gas component cannot follow these periodic orbits; instead, upon losing energy, the gas streams in elliptical trajectories at lower and lower radii, with their major axes leading more and more the periodic orbit, since the precession rate (estimated by - / 2 in the axisymmetric limit, for orbits near ILR, and by + / 2 near OLR) increases with decreasing radii in most of the disk. This regular shift forces the gas into a trailing spiral structure, from which the sense of the gravity torques can be easily derived. Inside corotation, the torques are negative, and the gas is driven inwards towards the inner Lindblad resonance (ILR). Inside ILR, and from the center, the precessing rate is increasing with radius, so that the gas pattern due to collisions will be a leading spiral, instead of a trailing one (see Figure 79). The gravity torques are positive, which also contributes to the accumulation of gas at the ILR ring. This situation is only inverted in the case of a central mass concentration (a black hole?), for which the precession rate - / 2 is monotonically increasing towards infinity with decreasing radii. Only then, the gravity torques will pull the gas towards the very center, and ``fuel'' the nucleus.

Figure 79. a) Without a central mass concentration, the gas winds up in a leading spiral inside the ILR ring; b) with a central mass concentration, the gas follows a trailing spiral structure.

What can form the active nucleus in the first place? It will be shown in the following sections that the accumulation of matter towards the center can produce a decoupling of a second bar inside the primary bar. This nuclear bar, and possibly other ones nested inside like Russian dolls, can take over the action of gravity torques to drive the gas to the nucleus, as first proposed by Shlosman et al. (1989).