N-body simulations of resonance rings in galactic disks

We have studied the formation of rings in the disks of galaxies by using tw o-dimensional N-body simulations where the gas component is modelled as dis sipatively colliding test particles. Our results support the standard hypot hesis that ring formation occurs when gas is driven to resonances by the gr avitational torque of a rotating stellar bar. When the bar is absent, a wea ker oval-shaped mode or a spiral mode can have the same effect. Typical loc ations of the rings are as follows: the outer rings are usually near the ou ter Lindblad resonances, the inner rings near the inner 4/1-resonance and t he 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 b e related to timescales of ring formation: the inner and nuclear rings usua lly form faster than the outer rings. The lack of inner and nuclear rings c an be related to the strength of the bar: in high amplitude cases, one or b oth 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 ri ngs. The potential outer ring region is often dominated by a slower spiral mode, which in principle could inhibit or delay ring formation. However, we foun d that when both the bar mode and the slower spiral mode coexist in the out er disk, there can be almost cyclic alternation between different outer rin g morphologies. In addition to the outer Lindblad resonance of the bar, cer tain 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 r ing. The presence of more than one mode could also affect the region of inn er or nuclear rings. This can explain part of the case in which the ring is misaligned with respect to the main bar component.