GAS-DYNAMICS IN NGC-4736

New test-particle simulations have been performed to study the secular evolution of the gaseous distribution in NGC 4736. We find that the d istribution of gas clouds can be understood in the frame of perturbati on induced by a nuclear oval potential, in addition to a spiral-like p otential. Our experiments show that both inner and outer rings are sta ble structures located at the inner Lindblad resonance (ILR) and the o uter Lindblad resonance(OLR), respectively, in agreement with the obse rvations of NGC 4736. One of our-simplified simulations indicates that both nuclear starburst and orbital resonance might be needed for keep ing the inner ring stable for a long period. We have introduced a symp lectic algorithm in the orbit integration. Its stronger stability allo ws us to adopt a rather large time step to save computational time. Su bstitution of a viscous force for cloud-cloud interactions proves adeq uate in the case of NGC 4736.