The Milky Way stellar halo out to 40 kpc : Squashed, broken but smooth

Understand

We introduce a new maximum likelihood method to model the density profile of Blue Horizontal Branch and Blue Straggler stars and apply it to the Sloan Digital Sky Survey Data Release 8 (DR8) photometric catalogue.

• There are a large number (20,000) of these tracers available over an impressive 14,000 square degrees in both Northern and Southern Galactic hemispheres, and they provide a robust measurement of the shape of the Milky Way stellar halo.
• After masking out stars in the vicinity of the Virgo Overdensity and the Sagittarius stream, the data are consistent with a smooth, oblate stellar halo with a density that follows a broken power-law.
• The best fitting model has an inner slope = 2.3 and an outer slope = 4.6, together with a break radius occurring at 27 kpc and a constant halo flattening (that is, ratio of minor axis to major axis) of q = 0.6.