David M. Nataf, Andrew P. Gould
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.
View original:
http://arxiv.org/abs/1112.1072
No comments:
Post a Comment