The Pristine survey

XXVIII. Journey to the Galactic outskirts: Mapping the outer halo red giant stars down to the very metal-poor end

¹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
² Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, STN CSC, Victoria, BC V8W 3P6, Canada
³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France
⁵ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany
⁶ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁷ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁸ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Santa Cruz de Tenerife, Spain
⁹ Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, 38205 La Laguna, Santa Cruz de Tenerife, Spain
¹⁰ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹¹ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias Universidad Diego Portales, Ejército 441, Santiago, Chile

^★★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 31 August 2024
Accepted: 5 October 2025

Abstract

Context. The outer Galactic halo remains relatively unexplored, particularly regarding its metallicity distribution, merger debris, and the population of very and extremely metal-poor ([Fe/H] < -2.5) stars.

Aims. Using photometric metallicities from the Pristine survey data release 1 (PDR1) and Pristine-Gaia synthetic (PGS) catalogue and Gaia DR3 astrometry, we constructed well-characterised samples of bright (G < 17.6) red giant branch (RGB) stars in the outer halo. With accurate distances, these samples enable studies of the halo’s metallicity distribution, accreted debris, and very metal-poor (VMP) substructures beyond 40 kpc.

Methods. We selected giants by excluding stars with reliable Gaia parallaxes in brightness ranges where dwarfs are measurable. Purity and completeness were validated against the Pristine spectroscopic training set. Distances were derived using BaSTI isochrone fitting combined with Pristine metallicity estimates.

Results. The photometric distances reach ~100 kpc (PDR1) and ~70 kpc (PGS), with typical uncertainties of 12% and scatter up to 20-40% compared to parallax- and StarHorse-based distances. The PDR1 sample provides a nearly unbiased metallicity-distance view, while the PGS sample offers an all-sky map, especially at the very metal-poor end. Using PDR1-giants, we traced the halo metallicity distribution function out to 101 kpc, fitting a three-component Gaussian mixture model. The most metal-poor component becomes increasingly dominant with distance, as beyond 50 kpc, 40-50% of the stars are very metal-poor ([Fe/H] < -2.0). With added radial velocities, we identified metallicity trends in integrals-of-motion space and investigated accreted debris. The PGS sample reveals substructures, including the Pisces Plume, where 41 VMP stars are linked to the Magellanic stream.

Conclusions. We publish two RGB catalogues: PDR1-giants (180 314 stars, with 10 096 very metal-poor candidates and 2096 beyond 40 kpc) and PGS-giants (2 420 898 stars, with 75 679 very metal-poor candidates and 267 beyond 40 kpc). These catalogues represent extensive resources for future outer halo studies.