Reconciling the Galactic Bulge Turnoff Age Discrepancy with Enhanced Helium Enrichment

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.