The Pristine survey - XXV. The very metal-poor Galaxy. Chemodynamics through the follow-up of the Pristine- Gaia synthetic catalogue
Authors: Viswanathan Akshara , Yuan Zhen , Ardern-Arentsen Anke , Starkenburg Else , Martin Nicolas F. , Youakim Kris , Ibata Rodrigo A. , Sestito Federico , Matsuno Tadafumi , Allende Prieto Carlos , Barwell Freya , Bayer Manuel , Doliva-Dolinsky Amandine , Fernandez-Alvar Emma , Galan-de Anta Pablo M. , Jhass Kiran , Longeard Nicolas , Maria Arroyo-Polonio Jose , Massana Pol , Montelius Martin , Rusterucci Samuel , Santos-Torres Judith , Thomas Guillaume F. , Vitali Sara , Wu Wenbo , Yarker Paige , Ye Xianhao , Aguado David S. , Gran Felipe , Navarro Julio 
Published: January 2025

The Pristine- Gaia synthetic catalogue of reliable photometric metallicities makes use of spectrophotometric information from Gaia DR3 XP spectra to calculate metallicity-sensitive CaHK magnitudes, which in turn provides photometric metallicities for ∼30 million FGK stars using the Pristine survey model and the survey's training sample. We performed the first low- to medium-resolution spectroscopic follow-up of bright (G<15) and distant (upto 35 kpc) very and extremely metal-poor (V/EMP Fe/H <-2.5) red giant branch stars from this catalogue--to evaluate the quality of the photometric metallicities and study the chemodynamics of these V/EMP stars. We used Isaac Newton Telescope/Intermediate Dispersion Spectrograph (INT/IDS) observations centred around the calcium triplet region ideal for V/EMP stars for this spectroscopic follow-up. We find that 76% of our stars indeed have Fe/H < -2.5 with these inferred spectroscopic metallicities, and only 3% are outliers with Fe/H > -2.0. We report a success rate of 77% and 38% in finding stars with Fe/H < -2.5 and -3.0, respectively. This is a huge improvement compared to the literature in the selection of V/EMP stars based on photometric metallicities and will allow for 10,000--20,000 homogeneously analysed EMP stars using the WEAVE survey follow-up of Pristine EMP candidates. Using kinematics, we categorised 20%, 46%, and 34% of the stars as being confined to the disc plane, or having inner and outer halo orbits, respectively. Based on their integrals-of-motion, we are able to associate these V/EMP stars with the metal-poor tail of the metallicity distribution functions of known accretion events such as the Gaia-Enceladus-Sausage, LMS-1/Wukong, Thamnos, Helmi streams, Sagittarius, Sequoia, and other retrograde mergers. For the stars that orbit close to the disc plane, we find that the prograde region with low vertical action is overdense with a significance of 4σ compared to its retrograde counterpart.

We also find three new (brightest) members of the most metal-poor stellar stream, C-19, one of which is 50^∘ from the main body of the stream. This is the first member of C-19 found at positive height above the disc plane. Our measured mean metallicity, velocity dispersion, and stream width are consistent with the literature, but our results favour a slightly farther distance (∼21.5 kpc) for the stream. With this work, we publish a catalogue (and 1D spectra) of 215 V/EMP stars from this first spectroscopic follow-up of the Pristine- Gaia synthetic catalogue of photometric metallicities and showcase the power of chemokinematic analysis of bright and distant red giant stars in the V/EMP end.