2025PASJ...77..178F

We select blue horizontal branch stars (BHBs) from the internal data release of the Hyper Suprime-Cam Subaru Strategic Program to reveal the global structure of the Milky Way (MW) stellar halo. The data are distributed over a $\sim$1100 deg$^2$ area in the range of $18.5\lt g\lt 24.5$ mag, so that candidate BHBs are detectable over a Galactocentric radius of $r \simeq 36-575$ kpc. In order to select the most likely BHBs by removing blue straggler stars and other contaminants in a statistically significant manner, we develop and apply an extensive Bayesian method, as described in Fukushima et al. (2019). Our sample can be fitted to either a single power-law profile with an index of $\alpha =4.11^{+0.18}_{-0.18}$ or a broken power-law profile with an index of $\alpha _{\rm in}=3.90^{+0.24}_{-0.30}$ at r below a broken radius of $r_{\rm b}=184^{+118}_{-66}$ kpc and a very steep slope of $\alpha _{\rm out}=9.1^{+6.8}_{-3.6}$ at $r\gt r_{\rm b}$; the statistical difference between these fitting profiles is small. Both profiles are found to show prolate shapes having axial ratios of $q=1.47^{+0.30}_{-0.33}$ and $1.56^{+0.34}_{-0.23}$, respectively. We also find a signature of the so-called “splashback radius” for the candidate BHBs, which can reach as large as $r \sim 575$ kpc, although it is still inconclusive owing to rather large distance errors in this faintest end of the sample. Our results suggest that the MW stellar halo consists of two overlapping components: the in situ inner halo showing a relatively steep radial density profile and the ex situ outer halo with a shallower profile, being characteristic of a component formed from accretion of small stellar systems.