SPECTRAL EVOLUTION OF STELLAR POPULATIONS USING ISOCHRONE SYNTHESIS

We combine the photometric model of isochrone synthesis recently published by Charlot & Bruzual with an updated library of stellar spectra to predict the spectral evolution of stellar populations with solar metallicity. The library of spectra assembled here supersedes other existing libraries (Bruzual; Guiderdoni & Rocca-Volmerange; Buzzoni) by its spectral range (extreme ultraviolet to far-infrared), its complete coverage of the color-magnitude diagram, and its inclusion of observed near-infrared spectra out to 2.56 mum. Also, the spectra are distributed on the stellar evolutionary tracks using optical/near-infrared color calibrations, as an improvement over models that used a single color of the effective temperature of the stars alone. The spectroscopic results obtained here confirm and extend the previous photometric predictions of the isochrone synthesis models while including the recent revision of evolutionary tracks for stars between 1.3 and 2.5 M. by their authors. Models with various time scales of star formation and a normal (Salpeter) initial mass function reproduce well the spectral and photometric properties from the ultraviolet to the near-infrared of nearby galaxies of various morphological types (from young irregular to older spiral and elliptical galaxies). We refine the stellar tracks to describe more accurately the evolution of hot planetary nebulae nuclei and white dwarfs and confirm in a self-consistent way the previous suggestion by Burstein et al. that these stars may account naturally for the far-ultraviolet radiation observed in ''quiescent'' early-type galaxies. The implications of the present models for the study of galaxy evolution are illustrated by the analysis of the redshift evolution of a few optical/near-infrared colors and spectral discontinuities for a representative range of model galaxies.