SPECTRO-PHOTOMETRIC EVOLUTION OF ELLIPTIC GALAXIES - III - INFALL MODELS WITH GRADIENTS IN MASS DENSITY AND STAR-FORMATION

In this study we present a simple model of elliptical galaxies aimed a t interpreting the gradients in colours and narrow band indices observ ed across these systems. Salient features of the model are the gradien ts in mass density and star formation and infall of primordial gas aim ed at simulating the collapse of a galaxy into the potential well of d ark matter. Adopting a multi-zone model we follow in detail the histor y of star formation, gas consumption, and chemical enrichment of the g alaxy and also allow for the occurrence of galactic winds according to the classical supernova (and stellar winds) energy deposit. The outli ne of the model, the time scale of gas accretion and rate of star form ation as a function of the galacto-centric distance in particular, see k to closely mimic the results from Tree-SPH dynamical models. Althoug h some specific ingredients of the model can be questioned from many p oints of view (of which we are well aware), the model has to be consid ered as a gross tool for exploring the consequences of different recip es of gas accretion and star formation in which the simple one-zone sc heme is abandoned. With the aid of this model we discuss the observati onal data on the gradients in metallicity, colours, and narrow band in dices across elliptical galaxies.