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

TL;DR

This study uses combined Pristine and SDSS photometry to identify blue horizontal branch stars in the Galactic halo, enabling detailed analysis of halo substructure and density profile with high accuracy.

Contribution

It introduces a new, cleaner sample of blue horizontal branch stars with high completeness and purity, improving the study of Galactic halo substructure and density profile.

Findings

Achieved 91% completeness and 93% purity in star classification.

Mapped multiple parts of the Sagittarius stream at large distances.

Found the Galactic halo follows a density profile with a slope of 3.5-4.0.

Abstract

We use the Pristine survey CaHK narrow-band photometry, combined with the 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% and a purity of 93% 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 of 3.5 - 4.0. As the relatively shallow SDSS u-band is the limiting factor in this technique, we foresee large potential for combining Pristine survey photometry with the much deeper u-band photometry from the Canada-France-Imaging Survey.