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

Akshara Viswanathan; Amanda Byström; Else Starkenburg; Anne Foppen; Jill Straat; Martin Montelius; Federico Sestito; Kim A. Venn; Camila Navarrete; Tadafumi Matsuno; Nicolas F. Martin; Guillaume F. Thomas; Anke Ardern-Arentsen; Giuseppina Battaglia; Morgan Fouesneau; Julio Navarro; Sara Vitali

doi:10.1051/0004-6361/202452073

arXiv · 2408.17250

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

Akshara Viswanathan·Amanda Byström·Else Starkenburg·Anne Foppen·Jill Straat·Martin Montelius·Federico Sestito·Kim A. Venn·Camila Navarrete·Tadafumi Matsuno·Nicolas F. Martin·Guillaume F. Thomas·Anke Ardern-Arentsen·Giuseppina Battaglia·Morgan Fouesneau·Julio Navarro·Sara Vitali

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Abstract

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. Aim: We utilize the Pristine survey's publicly available, Pristine data release 1 (PDR1) and Pristine-Gaia synthetic (PGS) catalogues of photometric metallicities, to select Red Giant Branch (RGB) stars in the outer halo. Methods: The RGB selection pipeline selects giants based on the absence of a well-measured parallax in the brightness range where dwarfs have reasonable parallax estimate from Gaia DR3 data. The photometric distances are calculated using a BaSTI-isochrone fitting code and the photometric metallicities. Results: Photometric distances derived from PDR1- and PGS-giants show typical uncertainties of 12% and a scatter of up to 20% and 40% respectively, when validated against inverted-parallax and Starhorse-code distances. 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 halo, especially in the VMP end. The PDR1-giants catalogue is used to study the metallicity distribution function of the halo out to ~100 kpc, using Gaussian Mixture Model decomposition. 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. Using the PGS-giants catalogue, we associate 41 stars tentatively to the stellar counterpart of the Magellanic stream in the VMP end, out to 70 kpc. Conclusions: We publish two RGB catalogues between -4.0<[Fe/H]<+0.1 with reliable photometric distances and metallicities.

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Akshara Viswanathan, Amanda Byström, Else Starkenburg, Anne Foppen, Jill Straat, Martin Montelius, Federico Sestito, Kim A. Venn, Camila Navarrete, Tadafumi Matsuno, Nicolas F. Martin, Guillaume F. Thomas, Anke Ardern-Arentsen, Giuseppina Battaglia, Morgan Fouesneau, Julio Navarro, Sara Vitali. 2024-08-30. The Pristine Survey -- XXVII. Journey to the Galactic outskirts -- Mapping the outer halo red giant stars down to the very metal-poor end. https://doi.org/10.1051/0004-6361/202452073

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