The Pristine survey

XXV. The very metal-poor Galaxy: Chemodynamics through the follow-up of the Pristine-Gaia synthetic catalogue^★

¹ Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
² Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
⁶ Dept. of Physics and Astronomy, University of Victoria, P. O. Box 3055, STN CSC, Victoria BC V8W 3P6, Canada
⁷ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12–14, 69120 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 Isaac Newton Group of Telescopes, Apartado 321, 38700 Santa Cruz de la Palma, Tenerife, Spain
¹¹ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK
¹² Department of Physics & Astronomy, University of Tampa, 401 West Kennedy Boulevard, Tampa, FL 33606, USA
¹³ Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA
¹⁴ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹⁵ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias Universidad Diego Portales, Ejército 441, Santiago, Chile
¹⁶ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
¹⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Nice, France

^★★ Corresponding author; astroakshara97@gmail.com

Received: 21 May 2024
Accepted: 23 July 2024

Abstract

Context. 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.

Aims. 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.

Methods. 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.

Results. 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.

Conclusions. 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.