The age structure of the Milky Way's halo

Authors :: Carollo, D.
Beers, T. C.
Placco, V. M.
Santucci, R. M.
Denissenkov, P.
Tissera, P. B.
Lentner, G.
Rossi, S.
Lee, Y. S.
Tumlinson, J.
Publication Year :: 2016
Abstract: 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 center (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 LambdaCDM 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.<br />Comment: Main Article: 12 pages, 4 figures; Supplemental Material: 4 pages, 1 figure. Nature Physics version: http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3874.html (Main article: 7 pages, 4 figures)