LUMINOUS RED STARS IN LOCAL GROUP DWARF ELLIPTICAL GALAXIES: AN INTERMEDIATE-AGE POPULATION?

摘要

In this Letter, we show that the optically bright stars above the tip of the first red giant branch (TRGB) in the color-magnitude diagrams (CMDs) of nearby dwarf elliptical (dE) galaxies, commonly interpreted as indication of the existence of an intermediate-age population, may be, in certain circumstances, an artifact of observational effects on a pure old population system. For this purpose, model CMDs have been computed, simulating the observational effects that can be found in different regions of a Local Group dE galaxy. The starting synthetic CMD represents a pure old system, with star formation extending only from 15 to 12 Gyr ago. Based on those model diagrams, we analyze the conclusions that a hypothetical ground-based observer might reach concerning the presence of a population of stars above the TRGB, which could be interpreted as an intermediate-age population, by using two age indicators extensively employed in the literature: (1) the bolometric magnitude at the tip of the asymptotic giant branch (AGB) phase and (2) the ratio of AGB to red giant branch (RGB) stars present in the CMD. Our analysis shows that, if observational effects are overlooked, then application of both methods would result in finding a fictitious intermediate-age population in the inner regions of the galaxy, where crowding is more severe.