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The Stellar Populations of Spiral Disks. II. Measuring and Modeling the Radial Distribution of Absorption Spectral Indices

The radial distributions of the Mg2 and Fe5270 Lick spectral indices have been measured to large radial distances on the disks of NGC 4303 and NGC 4535 using an imaging technique based on interference filters. These data, added to those of NGC 4321 previously published in this series, are used to constrain chemical (multiphase) evolutionary models for these galaxies. Because the integrated light of a stellar disk is a time average over the history of the galaxy weighted by the star formation rate, these constraints complement the information on chemical gradients provided by the study of H II regions, which by themselves can only provide the α-element abundance accumulated over the life of the galaxy. The agreement between the observations and the model predictions shown here lends confidence to the models, which are then used to describe the time evolution of galaxy parameters such as star formation rates, chemical gradients, and gradients in the mean age of the stellar population.