The Stellar Populations of Spiral Disks. Iii. Constraining Their Evolutionary Histories

We study the old problem of the uniqueness of chemical evolution models by analyzing a set of multiphase models for the galaxy NGC 4303 computed for a variety of plausible physical input parameters. We showed in Mollá et al. (1999) that multiphase chemical evolution models for the three Virgo cluster galaxies NGC 4303, NGC 4321 and NGC 4535, were able to reproduce the observed radial distributions of spectral indices Mg 2 ad Fe52. Chemical evolution models may, however, fit the present-epoch radial distributions with different star formation histories, thus we need to include time-dependent constraints to the problem. The two spectral indices above depend on the time–averaged history of star formation, but are in turn affected by the well known age-metallicity degeneracy which prevents the disentangling of age and metallicity for stellar populations, another uniqueness problem we also discuss. Our aim is to determine if the possible input parameters may be strongly constrained when both radial distributions (nebular and stellar) are used. In order to accomplish this we run a large number of models (500) for NGC 4303 varying the model input parameters. Less than 4 % of the models (19) fit the present day observational data within a region of 95% probability. The number of models reduces to a ∼ 1 % (6) when we also ask them to reproduce the time-averaged abundances represented by the spectral indices. Thus, by proving that only a small fraction of the models are favored in reproducing the present day radial abundance distributions and the spectral indices data simultaneously, we show that these spectral indices provide strong time-dependent additional constraints to the possible star formation and chemical histories of spiral disks. – 2 –