The Pristine Survey- XXVII. Journey to the Galactic outskirts: Mapping the outer halo red giant stars down to the very metal-poor end

Context. In the context of Galactic archaeology, the outer halo remains relatively unexplored with respect to its metallicity distribution, merger debris, and the abundance of known very/extremely metal-poor ([Fe/H]<–2.5) stars. Aims. We utilize the Pristine survey’s publicly available data, Pristine data release 1 (PDR1) and Pristine-Gaia synthetic (PGS) catalogues of photometric metallicities, to select Red Giant Branch (RGB) stars in the outer Galactic halo. Methods. The RGB selection pipeline selects giants based on the absence of a well-measured parallax in the brightness range where dwarfs have a reasonable parallax estimation from Gaia DR3 data. We ensure a good balance between purity and completeness by testing the method on the Pristine survey’s spectroscopic training sample. The photometric distances are calculated for the two samples of giants using a BaSTI-isochrone fitting code and the Pristine survey’s stellar metallicity estimates. Results. Photometric distances derived from PDR1- and PGS-giants show typical uncertainties of 12% and a scatter of upto 20% and 40% respectively, when validated against inverted-parallax and Starhorse-code distances, while going out to ∼100 and ∼70 kpc respectively. The PDR1-giants catalogue provides a low-to-no bias view of the metallicity structure versus distance compared to the PGS-giants catalogue (with a distance-metallicity selection bias), while the PGS-giants catalogue provides an all-sky view of the outer Galactic halo, especially in the VMP end. The PDR1-giants catalogue is used to study the metallicity distribution function (MDF) of the halo out to ∼100kpcanddownto[Fe/H]∼–4.0.Weshowthebias-correctedmetallicity distribution of the halo in six Galactocentric bins out to 101 kpc and fit a 3-component Gaussian mixture model to the underlying MDF. We see that as distance increases, the fractional contribution from the most metal-poor component increases. In the outer Galactic halo (d>50 kpc), 40-50% of the stars are very metal-poor (VMP, [Fe/H]<-2.0). Additionally, we use the PDR1-giants with radial velocities from spectroscopic surveys to map the metallicity view of the integrals-of-motion space where accreted dwarf galaxy debris conserve their orbital parameters for a long time. The PGS-giants catalogue is used to look for outer halo substructures such as the Pisces Plume overdensity where we associate 41 stars tentatively to the stellar counterpart of the Magellanic stream in the VMP end Conclusions. We publish two catalogues of RGB stars between-4.0<[Fe/H]<+0.1 from Pristine data release 1 and Pristine-Gaia synthetic photometric metallicities with reliable photometric distances inferred in this work. The PDR1-giants catalogue consists of 180,314 (111,305 with 6D phase-space data) giants out to ∼100 kpc, with 10,096 VMP candidate stars and 2,096 stars beyond 40 kpc, while the PGS-giants catalogue consists of 2,420,898 (1,706,006 with 6D phase-space data) giants out to ∼70 kpc, with 75,679 VMPcandidate stars and 267 VMP candidate stars beyond 40 kpc.