Galaxy formation and evolution - II. Energy balance, star formation and feedback

In this paper we present a critical discussion of the algorithms commonly u sed in N-body simulations of galaxy formation to deal with the energy equat ion governing heating and cooling, to model star formation and the star for mation rate, and to account for the energy feedback from stars. First, we p ropose our technique for solving the energy equation in the presence of hea ting and cooling, which includes some differences with respect to the stand ard semi-implicit techniques. Secondly, we examine the current criteria for the onset of the star formation activity. We suggest a new approach, in wh ich star formation is allowed to depend on the total mass density - baryoni c (gas and stars) and dark matter - of the system and on the metal-dependen t cooling efficiency. Thirdly, we check and discuss the separate effects of energy (and mass) feedback from several sources - namely supernovae, stell ar winds from massive stars and ultraviolet flux from the same objects. All the simulations are performed in the framework of the formation and evolut ion of a disc galaxy. We show that the inclusion of these physical phenomen a has a significant impact on the evolution of the galaxy models.