Structure of the Milky Way stellar halo out to its outer boundary with
  blue horizontal-branch stars
  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.
