Simulations of normal spiral galaxies

Results are presented of numerical simulations of normal isolated late type spiral galaxies. Specifically the galaxy NGC 628 is used as a template. The method employs a TREESPH code including stellar particles, gas particles, cooling and heating of the gas, star formation according to a Jeans criterion, and Supernova feedback. A regular spiral disc can be generated as an equilibrium situation of two opposing actions. On the one hand cooling and dissipation of the gas, on the other hand gas heating by the FUV field of young stars and SN mechanical forcing. The disc exhibits small and medium scale spiral structure of which the multiplicity increases as a function of radius. The theory of swing amplification can explain, both qualitatively and quantitatively, the emerging spiral structure. In addition, swing amplification predicts that the existence of a grand design m = 2 spiral is only possible if the disc is massive. The simulations show that the galaxy is then unstable to bar formation, confirming the result of Ostriker & Peebles (1973). The occurrence of this bar instability is further investigated. A general criterion is derived for the transition between bar stable and unstable, depending on disc mass contribution and on disc thickness. It seems that bar stability hardly depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. That demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared to the structure of some exemplary real galaxies. It is argued that the suggestion of Kormendy & Norman (1979) holds, that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy.