The Pristine Survey. XXX. Mapping substructures in the inner and outer halo with blue horizontal branch stars

Abstract

Blue horizontal branch (BHB) stars serve as good standard candles and kinematic tracers of the Milky Way halo. The halo displays various overdensities and kinematical substructures of different shapes and sizes. Our objective is to investigate the substructure in three dimensions and the bulk motion of the halo up to distances well exceeding the limit of the Gaia parallaxes. We selected BHB stars in the $\left(u-CaHK\right) _ 0$-$(g-r) _ 0$ colour-colour diagram with a hierarchical Bayesian mixture model of three populations: BHB stars, blue straggler stars, and outliers. Using $CaHK$ photometry derived from Gaia XP spectra and the Pristine survey, we provide both a shallow and a deep catalogue of BHB stars of 7,057 and 8,824 stars, respectively, where the deep catalogue is restricted to the deeper Pristine footprint compared to the shallower with Gaia XP. Using the shallow catalogue, we trace to larger distances 6D substructures identified earlier within a 2.5 kpc from the Sun, including accepted populations; namely, Gaia-Enceladus, Thamnos, Helmi streams, Sequoia, and other smaller halo substructures such as ED-3, ED-4, and ED-6. The deep catalogue provides reliable distance measurements for a more detailed 3D mapping of the Sagittarius stream and of the Pisces Overdensity. We add new candidate BHB stars with line-of-sight velocities to existing catalogues and confirm previously reported differences between the northern and southern Galactic hemispheres, likely due to the infall of the Large Magellanic Cloud. This research demonstrates that probabilistic photometric classification can effectively identify distant candidate BHB stars, and can provide catalogues of these stars that enhance our understanding of the 3D and 6D structures in the halo of the Milky Way.