To the Galactic Virial Radius with Hyper Suprime-Cam
  We exploit the exquisite, deep Hyper Suprime-Cam (HSC) imaging data to probe
the Galactic halo out to 200 kpc. Using the ~100 square degree, multi-band
photometry of the first HSC Wide survey data release, we identify blue
horizontal branch (BHB) stars beyond 50 kpc in the halo. The presence of the
Sagittarius (Sgr) stream in the HSC fields produces a notable excess of stars
at the apocentre of the leading arm (~50-60 kpc). For fields excluding Sgr, the
BHB counts are consistent with a continuation of a -4 power-law from the inner
halo. However, we find that the majority of the non-Sgr BHB stars beyond 50 kpc
reside in one 27 square degree HSC field called "VVDS". Curiously, this field
is located close to the Magellanic plane, and we hypothesize that the excess of
stars between 50 and 200 kpc could be associated with distant Magellanic
debris. Indeed, without the VVDS, there are very few BHBs in the remaining
portions of the Galaxy probed by the HSC. Accordingly, this scarcity of tracers
is consistent with a significant decline in stellar density beyond 50 kpc, with
a power-law of -4 or steeper.
