Chemical evolution of galaxies -: I.: A composition-dependent SPH model for chemical evolution and cooling

We describe an smooth particle hydrodynamics (SPH) model for chemical enrichment and radiative cooling in cosmological simulations of structure formation. This model includes: (i) the delayed gas restitution from stars by means of a probabilistic approach designed to reduce the statistical noise and, hence, to allow for the study of the inner chemical structure of objects with moderately high numbers of particles; (ii) the full dependence of metal production on the detailed chemical composition of stellar particles by using, for the first time in SPH codes, the Q(ij) matrix formalism that relates each nucleosynthetic product to its sources and (iii) the full dependence of radiative cooling on the detailed chemical composition of gas particles, achieved through a fast algorithm using a new metallicity parameter zeta(T) that gives the weight of each element on the total cooling function. The resolution effects and the results obtained from this SPH chemical model have been tested by comparing its predictions in different problems with known theoretical solutions. We also present some preliminary results on the chemical properties of elliptical galaxies found in self-consistent cosmological simulations. Such simulations show that the above zeta-cooling method is important to prevent an overestimation of the metallicity-dependent cooling rate, whereas the Q(ij) formalism is important to prevent a significant underestimation of the [alpha/Fe] ratio in simulated galaxy-like objects.