Uncertainties of Synthetic Integrated Colors as Age Indicators

Abstract

We investigate the uncertainties in the synthetic integrated colors of simple stellar populations , currently the most popular method of estimating the ages of unresolved stellar systems. Three types of uncertainties studied here originate from the stellar models, the population synthesis techniques, and the stellar spectral libraries. Despite some skepticism, synthetic colors appear to be reliable age indicators as long as they are used for select age ranges. Rest-frame optical colors are good age indicators at ages 2-7 Gyr, mainly due to the clear redward evolution of hydrogen-burning stars (main-sequence stars and red giants). At ages sufficiently large to produce hot horizontal-branch stars, the UV-to-optical colors may provide an alternative means for measuring ages. This implies that one can use integrated colors as age indicators for globular clusters in nearby external galaxies and perhaps even for high-redshift galaxies that are passively evolving. Such studies may provide important tests of various galaxy formation scenarios.