The Pristine survey - VII. A cleaner view of the Galactic outer halo using blue horizontal branch stars

We use the Pristine survey CaHK narrow-band photometry, combined with the Sloan Digital Sky Survey (SDSS)ugr photometry, to provide a cleaner sample of blue horizontal branch stars in the Galactic halo out to large distances. We demonstrate a completeness of 91 per cent and a purity of93 per cent with respect to available spectroscopic classifications. We subsequently use our new clean sample of these standard candles to investigate the substructure in the Galactic halo over the Pristine footprint. Among other features, this allows for a careful tracing of multiple parts of the Sagittarius stream, providing a measurement independent from other tracers used and reaching larger distances.Moreover, we demonstrate with this clean and complete sample that the halo follows a density profile with a negative power-law slope of3.5-4.0. As the relatively shallow SDSS u band is the limiting factor int his technique, we foresee large potential for combining Pristine survey photometry with the much deeper u-band photometry from the Canada-France-Imaging Survey.