A NEW CHEMO-EVOLUTIONARY POPULATION SYNTHESIS MODEL FOR EARLY-TYPE GALAXIES .1. THEORETICAL BASIS

We have developed a new stellar population synthesis model designed to study early-type galaxies. It provides optical and near-infrared colo rs, and line indices for 25 absorption lines. It can synthesize single age, single metallicity stellar populations, or follow the galaxy thr ough its evolution from an initial gas cloud to the present time. The model incorporates the new isochrones of the Padova group and the late st stellar spectral libraries. We have applied our model to new data f or a set of three early-type galaxies to find out whether these can be fitted using single-age old metal-rich stellar populations, as is nor mal practice when one uses other stellar models of this kind. The mode l is extensively compared with previous ones in the literature to esta blish its accuracy as well as the accuracy of this kind of models in g eneral. Using the evolutionary version of the model we find that we ca nnot fit the most metal-rich elliptical galaxies if we keep the IMF co nstant and do not allow infall of gas. We do, however, reproduce the r esults of Arimoto & Yoshii for the evolution of the gas, and produce c olors, and, for the first time with this type of models, absorption li ne strengths. It is in fact possible to fit the data for the elliptica l galaxies by varying the IMF with time. Our numerical model is in goo d broad agreement with the analytical simple model. We prefer, however , to calculate the evolution of the gas numerically instead of using t he simple model, since it offers more flexibility, and even improved i nsight, when comparing with observations. In the present paper we desc ribe the model, and compare a few key observables with new data for th ree early-type standard galaxies. However, the data, as well as our fi ts, will be discussed in much more detail in a second paper (by Vazdek is et al.), where some conclusions will be drawn about elliptical gala xies on the basis of this model.