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N-body simulations of resonance rings in galactic disks

We have studied the formation of rings in the disks of galaxies by using two-dimensional N-body simulations where the gas component is modelled as dissipatively colliding test particles. Our results support the standard hypothesis that ring formation occurs when gas is driven to resonances by the gravitational torque of a rotating stellar bar. When the bar is absent, a weaker oval-shaped mode or a spiral mode can have the same effect. Typical locations of the rings are as follows: the outer rings are usually near the outer Lindblad resonances, the inner rings near the inner 4/1-resonance and the nuclear rings near the inner Lindblad resonances. However, we have also found a few exceptions to these rules. We also have studied why a significant fraction of barred galaxies lack one, two or all ring types. Our models suggest that the absence of rings may be related to timescales of ring formation: the inner and nuclear rings usually form faster than the outer rings. The lack of inner and nuclear rings can be related to the strength of the bar: in high amplitude cases, one or both of these ring types are absent. Also, bars may rotate fast enough such that they lack the inner Lindblad resonance and thus cannot form nuclear rings. The potential outer ring region is often dominated by a slower spiral mode, which in principle could inhibit or delay ring formation. However, we found that when both the bar mode and the slower spiral mode coexist in the outer disk, there can be almost cyclic alternation between different outer ring morphologies. In addition to the outer Lindblad resonance of the bar, certain resonances of the slower mode can also exist near the ring radius. The deceleration of the bar rotation rate and the corresponding change in the resonance positions did not inhibit ring formation or destroy an existing ring. The presence of more than one mode could also affect the region of inner or nuclear rings. This can explain part of the case in which the ring is misaligned with respect to the main bar component.