Reconciling the Galactic Bulge Turnoff Age Discrepancy with Enhanced Helium Enrichment

TL;DR

This paper proposes that an elevated helium enrichment in the Galactic bulge explains the age discrepancy between spectroscopic and photometric measurements, suggesting a mean age of about 10 billion years and implications for stellar models.

Contribution

It introduces a helium enrichment model that reconciles age measurements of the Galactic bulge, challenging standard isochrone assumptions and proposing new theoretical tools.

Findings

Helium enrichment can explain the age discrepancy.

Upper bound on helium enrichment parameter is approximately 5.0.

Bulge's mean age is about 10 Gyr.

Abstract

We show that the factor $\sim$2 discrepancy between spectroscopic and photometric age determinations of the Galactic bulge main-sequence turnoff can be naturally explained by positing an elevated helium enrichment for the bulge relative to that assumed by standard isochrones. We obtain an upper bound on the helium enrichment parameter of the bulge $({\Delta}Y/{\Delta}Z)_{\rm{Bulge}} \lesssim 5.0$ given the requirement that the spectroscopic and photometric ages be consistent and the limiting condition of instantaneous star formation. The corresponding mean age for the bulge is $t_{\rm{Bulge}} \approx 10$ Gyr. We discuss phenomenological evidence that the bulge may have had a chemical evolution that is distinct from the solar neighborhood in this manner, and we make several testable predictions. Should this emerging picture of the bulge as helium-enhanced hold, it will require the development of new isochrones, new model atmospheres, and modified analysis and cosmological interpretation of the integrated light of other bulges and elliptical galaxies.