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

TL;DR

This paper introduces an enhanced population synthesis model that accounts for helium enrichment effects on surface brightness fluctuations, which can help detect He-rich populations in distant unresolved stellar systems.

Contribution

The study develops a new SBF model for He-enriched populations, highlighting their impact on SBF magnitudes and proposing methods to identify He-rich populations in galaxies.

Findings

He-rich populations cause at least 0.3 mag change in SBF magnitudes.

Normal-He SBF models agree with existing models.

UV and optical SBFs can detect He-rich populations beyond 20 Mpc.

Abstract

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 $\sim$0.3~mag even in $I$- and near-IR 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 ${\rm Mg}_2$ and ${\rm H}\beta$, the UV and optical SBFs can readily detect underlying He-rich populations in unresolved stellar systems at a distance out to $\gtrsim 20$\,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.