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

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.

14 pages, 11 figures. Accepted for publication in MNRAS

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• Mapping the anisotropic Galactic stellar halo with Blue Horizontal Branch stars
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• Clean catalogues of blue horizontal-branch stars using Gaia EDR3
• The Far-Away Blues: Exploring the Furthest Extents of the Boötes I Ultra Faint Dwarf Galaxy
• A LAMOST BHB Catalog and Kinematics Therein I: Catalog and Halo Properties
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• A Pristine-UNIONS view on the Galaxy: Kinematics of the distant spur feature of the Sagittarius stream traced by Blue Horizontal Branch stars
• The Pristine Survey -- XXVII. Journey to the Galactic outskirts -- Mapping the outer halo red giant stars down to the very metal-poor end
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