On the relative ages of the α-rich and α-poor stellar populations in the Galactic halo

We study the ages of alpha-rich and alpha-poor stars in the halo using a sample of F and G dwarfs from the Sloan Digital Sky Survey (SDSS). To separate stars based on [alpha/Fe], we have developed a new semi-empirical spectral index-based method and applied it to the low-resolution, moderate signal-to-noise SDSS spectra. The method can be used to estimate the [alpha/Fe] directly providing a new and widely applicable way to estimate [alpha/Fe] from low-resolution spectra. We measured the main-sequence turnoff temperature and combined it with the metallicities and a set of isochrones to estimate the age of the alpha-rich and alpha-poor populations in our sample. We found all stars appear to be older than 8 Gyr confirming the idea that the Galactic halo was formed very early on. A bifurcation appears in the age-metallicity relation such that in the low-metallicity regime the alpha-rich and alpha-poor populations are coeval while in the high-metallicity regime the alpha-rich population is older than the alpha-poor population. Our results indicate the alpha-rich halo population, which has shallow age-metallicity relation, was formed in a rapid event with high star formation, while the alpha-poor stars were formed in an environment with a slower chemical evolution time-scale.