Final results for mapping the Milky Way's stellar halo with blue horizontal branch stars selected from the Subaru Hyper Suprime-Cam survey.

This article focuses on mapping the global structure of the Milky Way (MW) stellar halo using blue horizontal branch stars (BHBs) selected from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) data. Applying an extensive Bayesian method to minimize contamination, the study analyzes BHBs over a Galactocentric radius range of approximately 36–575 kpc, finding that their spatial distribution is well described by either a single power-law density profile with an index of about 4.11 or a broken power-law with an inner slope near 3.90 and a much steeper outer slope beyond ~184 kpc; both profiles indicate a prolate halo shape. The results suggest the MW stellar halo comprises two overlapping components: an in situ inner halo with a relatively steep density profile and an ex situ outer halo formed from accreted smaller stellar systems, consistent with predictions from galaxy formation simulations. While a tentative signature of the "splashback radius"—a physical boundary marking the halo's edge—is observed near 575 kpc, the evidence remains inconclusive due to data uncertainties. Comparisons with other halo tracers and surveys reveal differences in density slopes, highlighting the need for deeper and wider future observations to better understand the MW's assembly history.