Mapping the anisotropic Galactic stellar halo with blue horizontal branch stars

We used Legacy Survey photometric data to probe the stellar halo in multiple directions of the sky using a probabilistic methodology to identify blue horizontal branch (BHB) stars. The measured average radial density profile follows a double power law in the range 5 < r(gc)/kpc < 120, with a density break at r(gc) approximate to 20 kpc. This description, however, falls short, depending on the chosen line of sight, with some regions showing no signature of a break in the profile and a wide range of density slopes, such as an outer slope -5.5 less than or similar to alpha out less than or similar to -4, pointing towards a highly anisotropic stellar halo. This explains, in part, the wide range of density profiles reported in the literature owing to different tracers and sky coverage. Using our detailed 3D stellar halo density map, we quantified the shape of the Pisces overdensity associated with the transient wake response of the Galaxy's (dark) halo to the Large Magellanic Cloud (LMC). Measured in the LMC's coordinate system, Pisces stands above the background, is 60 degrees long and 25 degrees wide, and is aligned with the LMC's orbit. This would correspond to a wake width of similar to 32 kpc at similar to 70 kpc. We do not find a statistically significant signature of the collective response in density as previously reported in the literature measured with K giant stars, despite our larger numbers. We release the catalogue constructed in this study with 95 446 possible BHB stars and their BHB probability.