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.

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Figure 1: Chronographic map for photometrically selected BHB stars from the SDSS.
Figure 2: Chronographic map in the region of the southern (leading/trailing) arm of the Sagittarius Stream.
Figure 3: Colour (age) distribution of accepted pixels (containing at least 10 stars) for different ranges of the vertical distance, Z.
Figure 4: Colour (age) gradient in the vertical distance and in the radial distance directions.


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D.C., T.C.B., V.M.P. and G.L. acknowledge partial support for this work from grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. Y.S.L. acknowledges support provided by the National Research Foundation of Korea to the Center for Galaxy Evolution Research (No. 2010-0027910) and partial support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2015R1C1A1A02036658). R.M.S. and S.R. acknowledge CAPES (PROEX), CNPq, PRPG/USP, FAPESP and INCT-A funding. P.D. acknowledges partial funding from a Natural Sciences and Engineering Research Council of Canada grant to D. VandenBerg. P.B.T. acknowledges partial support from PICT-959-2011, Fondecyt-113350 and MUN-UNAB projects.

Author information

D.C., T.C.B., V.M.P., R.M.S., G.L. and Y.S.L. performed the analysis and interpretations of the observations. The chronographic maps were assembled on the basis of graphical techniques developed by V.M.P. P.D. carried out modelling of the mapping of BHB colours to age estimates. D.C., T.C.B., P.B.T. and J.T. carried out comparisons of the results with expectations from numerical simulations of galaxy formation. All authors discussed the results and commented on the manuscript.

Correspondence to D. Carollo.

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Carollo, D., Beers, T., Placco, V. et al. The age structure of the Milky Way’s halo. Nature Phys 12, 1170–1176 (2016).

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