The effect of helium-enhanced stellar populations on the ultraviolet-upturn phenomenon of early-type galaxies

Recent observations and modeling of globular clusters (GCs) with multiple populations strongly indicate the presence of super-helium-rich subpopulations in old stellar systems. Motivated by this, we have constructed new population synthesis models with and without helium-enhanced subpopulations to investigate their impact on the UV-upturn phenomenon of quiescent early-type galaxies (ETGs). We find that our models with helium-enhanced subpopulations can naturally reproduce the strong UV-upturns observed in giant elliptical galaxies assuming an age similar to that of old GCs in the Milky Way. The major source of far-UV (FUV) flux, in this model, is relatively metal-poor and helium-enhanced hot horizontal-branch stars and their progeny. The Burstein etal. relation of the FUV - V color with metallicity is also explained either by the variation of the fraction of helium-enhanced subpopulations or by the spread in mean age of stellar populations in ETGs.