To the Galactic Virial Radius with Hyper Suprime-Cam

Abstract: 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.