The age structure of the Milky Way's halo

We present a new, high-resolution chronographic (age) map of the Milky Way´s halo, based on the inferred ages of ∼130,000 field blue horizontal-branch (BHB) stars with photometry from the Sloan Digital Sky Survey. Our map exhibits a strong central concentration of BHB stars with ages greater than 12 Gyr, extending up to ∼15 kpc from the Galactic Centre (reaching close to the solar vicinity), and a decrease in the mean ages of field stars with distance by 1-1.5 Gyr out to ∼45-50 kpc, along with an apparent increase of the dispersion of stellar ages, and numerous known (and previously unknown) resolved over-densities and debris streams, including the Sagittarius Stream. These results agree with expectations from modern lambda cold dark matter cosmological simulations, and support the existence of a dual (inner/outer) halo system, punctuated by the presence of over-densities and debris streams that have not yet completely phase-space mixed.Fil: Carollo, D.. University of Notre Dame; Estados UnidosFil: Beers, T. C.. University of Notre Dame; Estados UnidosFil: Placco, V. M.. University of Notre Dame; Estados UnidosFil: Santucci, R. M.. Universidade de Sao Paulo; BrasilFil: Denissenkov, P.. University Of Victoria; CanadáFil: Tissera, Patricia Beatriz. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Lentner, G.. University of Notre Dame; Estados UnidosFil: Rossi, S.. Universidade de Sao Paulo; BrasilFil: Lee, Y. S.. Chungnam National University; Corea del SurFil: Tumlinson, J.. Space Telescope Science Institute; Estados Unido