  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. 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 a
grand design can only be generated by a central bar or by tidal forces
resulting from an encounter with another galaxy.
