Effects of young stellar populations on the determination of ages and metallicities of the dominating stellar populations of galaxies

Context. The ages and metallicities of the dominating stellar populations of galaxies without current star formation (e.g., early type galaxies) are usually estimated by comparing the observed absorption line indices or colors to predictions of some simple stellar population models. However, some studies show that there is recent star formation in galaxies, including early type galaxies. This suggests that we possibly do not obtain accurate ages and metallicities of the dominating stellar populations of galaxies. This is obvious when we estimate the stellar ages and metallicities by colors of galaxies, because the youngest populations dominate the light and make the fitted stellar populations younger and more metal rich. Aims. We plan to study how young populations in composite stellar populations affect the colors of star systems and to analyze how the stellar ages and metallicities derived from colors possibly deviate from those of the dominating populations of composite star systems. Methods. The techniques of stellar population synthesis and Monte Carlo process are used in this work. Results. It is found that the age and mass fraction of the young population of a composite stellar population affect colors of the system significantly, but the former is stronger than the latter. In addition, our results show that the stellar ages and metallicities derived directly from a pair of colors are about 2.14 Gyr younger while 0.0027 more metal rich than those of the dominating populations of composite stellar systems on average. Some possible distributions of the differences between stellar ages and metallicities determined by colors, and those of the dominating stellar populations of composite stellar systems are presented in this work. Some possible distributions of the differences between colors of composite stellar populations and those of their dominating stellar populations are also shown in this paper. Stellar ages and metallicities measured by colors and line strength indices are compared in the work, with a sample of 18 galaxies. Furthermore, the YSPs may affect the Fundamental Plane and Kormendy relation of early type galaxies.