Galaxy Formation and Evolution. II. Energy Balance, Star Formation and Feed-back

In this paper we present a critical discussion of the algorithms commonly used in N-body simulations of Galaxy Formation to deal with the energy equation governing heating and cooling, to model star formation and the star formation rate, and to account for energy feed-back from stars. First, we propose our technique for solving the energy equation in presence of heating and cooling, which includes some difference with respect to the standard semi-implicit technique. Second, we examine the current criteria for the onset of the star formation activity. We suggest a new approach, in which star formaiton is let depend on the total mass density - baryonic (gas and stars) and dark matter - of the system and on the metal-dependent cooling efficiency. Third, we check and discuss the separate effects of energy (and mass) feed-back from several sources - namely supernovae, stellar winds from massive stars, and UV flux from the same objects. All the simulations are performed in the framework of the formation and evolution of a disk galaxy. We show that the inclusion of these physical phenomena has a signigicant impact on the evolution of the galaxy model.