Simulations of normal spiral galaxies

Results from numerical simulations of normal isolated late-type spiral galaxies are presented; 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 far-ultraviolet field of young stars and supernova 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. The occurrence of this bar instability is further investigated. A general criterion is derived for the transition between a stable and an unstable bar, depending on the disc mass contribution and the on-disc thickness. It seems that bar stability barely depends on the presence of gas. A detailed quantitative analysis is made of the emerging spiral structure and a comparison is made with observations. This demonstrates that the structure of the numerical isolated galaxies is not as strong and has a larger multiplicity compared with the structure of some exemplary real galaxies. It is argued that the suggestion of Kormendy & Norman holds, i.e. that a grand design can only be generated by a central bar or by tidal forces resulting from an encounter with another galaxy.