We report on the global structure of the Milky Way ( MW ) stellar halo up to its outer boundary based on the analysis of blue-horizontal branch stars ( BHBs ) .
These halo tracers are extracted from the ( g,r,i,z ) band multi-photometry in the internal data release of the on-going Hyper Suprime-Cam Subaru Strategic Program ( HSC-SSP ) surveyed over \sim 550 deg ^ { 2 } area .
In order to select most likely BHBs by removing blue straggler stars ( BSs ) and other contamination in a statistically significant manner , we have developed and applied an extensive Bayesian method , instead of the simple color cuts adopted in our previous work , where each of the template BHBs and non-BHBs obtained from the available catalogs is represented as a mixture of multiple Gaussian distributions in the color-color diagrams .
We found from the candidate BHBs in the range of 18.5 < g < 23.5 mag that the radial density distribution over a Galactocentric radius of r = 36 - 360 kpc can be approximated as a single power-law profile with an index of \alpha = 3.74 ^ { +0.21 } _ { -0.22 } or a broken power-law profile with an index of \alpha _ { in } = 2.92 ^ { +0.33 } _ { -0.33 } at r below a broken radius of r _ { b } = 160 ^ { +18 } _ { -19 } kpc and a very steep slope of \alpha _ { out } = 15.0 ^ { +3.7 } _ { -4.5 } at r > r _ { b } .
The latter profile with a prolate shape having an axial ratio of q = 1.72 ^ { +0.44 } _ { -0.28 } is most likely and this halo may hold a rather sharp boundary at r \simeq 160 kpc .
The slopes of the halo density profiles are compared with those from the suite of hydrodynamical simulations for the formation of stellar halos .
This comparison suggests that the MW stellar halo may consist of the two overlapping components : the in situ . inner halo as probed by RR Lyrae stars showing a relatively steep radial density profile and the ex situ . outer halo with a shallow profile probed by BHBs here , which is made by accretion of small stellar systems .