We present the structure of the Milky Way stellar halo beyond Galactocentric distances of r = 50 kpc traced by blue horizontal-branch ( BHB ) stars , which are extracted from the survey data in the Hyper Suprime-Cam Subaru Strategic Program ( HSC-SSP ) .
We select BHB candidates based on ( g,r,i,z ) photometry , where the z -band is on the Paschen series and the colors that involve the z -band are sensitive to surface gravity .
About 450 BHB candidates are identified between r = 50 kpc and 300 kpc , most of which are beyond the reach of previous large surveys including the Sloan Digital Sky Survey .
We find that the global structure of the stellar halo in this range has substructures , which are especially remarkable in the GAMA15H and XMM-LSS fields in the HSC-SSP .
We find that the stellar halo can be fitted to a single power-law density profile with an index of \alpha \simeq 3.3 ( 3.5 ) with ( without ) these fields and its global axial ratio is q \simeq 2.2 ( 1.3 ) .
Thus , the stellar halo may be significantly disturbed and be made in a prolate form by halo substructures , perhaps associated with the Sagittarius stream in its extension beyond r \sim 100 kpc .
For a broken power-law model allowing different power-law indices inside/outside a break radius , we obtain a steep power-law slope of \alpha \sim 5 outside a break radius of \simeq 100 kpc ( 200 kpc ) for the case with ( without ) GAMA15H and XMM-LSS .
This radius of 200 kpc might be as close as a halo boundary if there is any , although larger BHB sample is required from further HSC-SSP survey to increase its statistical significance .