Star formation in N-body simulations .1. The impact of the stellar ultraviolet radiation on star formation

We present numerical simulations of isolated disk galaxies including gas dynamics and star formation. The gas is allowed to cool to 10 K, while heating of the gas is provided by the far-ultraviolet flux of all stars. Stars are allowed to form from the gas according to a Jeans instability criterion: gas is unstable when the Jeans mass is smaller than a critical mass, and stars form as soon as the gas remains unstable longer than the collapse time. With these ingredients we are able to create a two-phase interstellar medium and our model gives realistic star formation rates (SFRs).We investigate the influence of free parameters on the star formation. In order of decreasing importance these are: ionization fraction of the gas (determines the cooling properties), initial mass function (controls the heat input for the gas), collapse time for molecular clouds, and star formation efficiency.In the simulations the star formation quickly settles a kind of thermal equilibrium of the ISM. This result strongly favours the self-regulating mechanism for star formation.The model yields a Schmidt law power dependence of the SFR on gas density (SFR proportional to p(n)) with index n approximate to 1.3, in good agreement with observations.The simulations show that star formation can only occur in the mid-plane of the galaxy, where the gas is dense enough to cool below 100 K. The gas above the plane and outside approximately 6 radial scale lengths is always warm (T > 8000 K), heated by the stellar photons. It suggests that radial truncation of stellar disks is a thermal rather than a dynamical process.Flocculent spiral structure is generally found in the cold gas and consequently also in the young stellar population. It suggests that flocculent spirals are due to the dissipational nature of gas.