SIMBAD references

New test-particle simulations have been performed to study the secular evolution of the gaseous distribution in NGC 4736. We find that the distribution of gas clouds can be understood in the frame of perturbation induced by a nuclear oval potential, in addition to a spiral-like potential. Our experiments show that both inner and outer rings are stable structures located at the inner Lindblad resonance (ILR) and the outer Lindblad resonance(OLR), respectively, in agreement with the observations of NGC 4736. One of our simplified simulations indicates that both nuclear starburst and orbital resonance might be needed for keeping the inner ring stable for a long period. We have introduced a symplectic algorithm in the orbit integration. Its stronger stability allows us to adopt a rather large time step to save computational time. Substitution of a viscous force for cloud-cloud interactions proves adequate in the case of NGC 4736.