Yonsei Evolutionary Population Synthesis (YEPS) Model. III. Surface Brightness Fluctuation of Normal and Helium-enhanced Simple Stellar Populations

We present an evolutionary population synthesis model of the surface brightness fluctuation (SBF) for normal and He-enriched simple stellar populations (SSPs). While our SBF model for the normal He population agrees with other existing models, the He-rich population, containing hotter horizontal branch stars and brighter red clump stars than the normal He population, entails a substantial change in the SBF of SSPs. We show that the SBF magnitudes are affected by He-rich populations at least ∼0.3 mag even in I- and near-infrared bands at given colors, from which the SBF-based distances are often derived. Due to uncertainties both in observations and models, however, the SBFs of Galactic globular clusters and early-type galaxies do not allow verifying the He-enriched model. We propose that when combined with independent metallicity and age indicators such as Mg₂ and Hβ, the UV and optical SBFs can readily detect underlying He-rich populations in unresolved stellar systems at a distance out to ≳20 Mpc. A full set of the spectro-photometric and SBF data for SSPs from the Yonsei Evolutionary Population Synthesis (YEPS) model is available for download at http://cosmic.yonsei.ac.kr/YEPS.htm.