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

TL;DR

This study demonstrates that helium-enhanced stellar populations can explain the strong ultraviolet-upturn observed in early-type galaxies, aligning models with recent globular cluster observations.

Contribution

The paper introduces new population synthesis models incorporating helium-enhanced subpopulations to explain the UV-upturn phenomenon in early-type galaxies.

Findings

Helium-enhanced subpopulations reproduce observed UV-upturns in ellipticals.

FUV flux mainly from helium-rich hot horizontal branch stars.

Variation in helium subpopulations or age spread explains UV-color metallicity relation.

Abstract

Recent observations and modeling of globular clusters 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. 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 globular clusters 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 et al. (1988) relation of the F U V - 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 early-type galaxies.