Synthetic stellar populations: single stellar populations, stellar interior models and primordial protogalaxies

Abstract

ABSTRACT We present a new set of stellar interior and synthesis models for predicting the inte-grated emission from stellar populations in star clusters and galaxies of arbitrary ageand metallicity. This work differs from existing spectral synthesis codes in a number ofimportant ways, namely (1) the incorporation of new stellar evolutionary tracks, withsufficient resolution in mass to sample rapid stages of stellar evolution; (2) a phys-ically consistent treatment of evolution in the HR diagram, including the approachto the main sequence and the effects of mass loss on the giant and horizontal-branchphases. Unlike several existing models, ours yield consistent ages when used to datea coeval stellar population from a wide range of spectral features and colour indexes.We use Hipparcos data to support the validity of our new evolutionary tracks. Werigorously discuss degeneracies in the age-metallicity plane and show that inclusion ofspectral features blueward of 4500 ˚A suffices to break any remaining degeneracy andthat with moderate S/N spectra (10 per 20˚A resolution element) age and metallicityare not degenerate. We also study sources of systematic errors in deriving the ageof a single stellar population and conclude that they are not larger than 10 − 15%.We illustrate the use of single stellar populations by predicting the colors of primor-dial proto-galaxies and show that one can first find them and then deduce the formof the IMF for the early generation of stars in the universe. Finally, we provide ac-curate analytic fitting formulas for ultra fast computation of colors of single stellarpopulations.Key words: galaxies: stellar populations; stars: stellar evolution