The Pristine Dwarf-Galaxy survey

VI. A VLT/FLAMES spectroscopic study of the dwarf galaxy Boötes II

¹ Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Versoix, Switzerland
² IFCA, Instituto de Física de Cantabria (UC-CSIC), Av. de Los Castros s/n, 39005 Santander, Spain
³ GEPI, Observatoire de Paris, Université PSL, CNRS, Place Jules Janssen, F-92195 Meudon, France
⁴ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38206 La Laguna, Santa Cruz de Tenerife, Spain
⁵ Universidad de La Laguna, Avda. Astrofísico Francisco Sánchez, 38205 La Laguna, Santa Cruz de Tenerife, Spain
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁷ Myrspoven AB, Västgötagatan 1, 11827 Stockholm, Sweden
⁸ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France
⁹ Dept. of Physics and Astronomy, University of Victoria, PO Box 3055, STN CSC Victoria, BC V8W 3P6, Canada
¹⁰ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield AL10 9AB, UK

^⋆ Corresponding author.

Received: 20 September 2024
Accepted: 4 April 2025

Abstract

Aims. The Milky Way has a large population of dwarf galaxy satellites. Their properties are sensitive to both cosmology and the physical processes underlying galaxy formation, but these properties are still not properly characterised for the entire satellite population.

Methods. We aim to provide the most accurate systemic dynamical and metallicity properties of the dwarf galaxy Boötes II (Boo II).

Results. We use a new spectroscopic sample of 39 stars in the field of Boo II (heliocentric distance of ∼66 kpc) with data from the Fiber Large Array Multi Element Spectrograph (FLAMES) mounted on the Very Large Telescope (VLT). The target selection is based on a combination of broadband photometry, proper motions from Gaia, and the metallicity-sensitive narrow-band photometry from the Pristine survey that is ideal for removing obvious Milky Way contaminants.

Conclusions. We found nine new members, including five also found by recent works in the literature, and the farthest member to date (5.7 half-light radii from Boo II centroid), extending the spectroscopic spatial coverage of this system. Our metallicity measurements based on the Calcium triplet lines leads to the detection of the two first Extremely Metal-poor stars ([Fe/H] < −3.0) in Boo II. Combining this new dataset with literature data refines Boo II’s velocity dispersion (5.6_−1.1^+1.8 km s⁻¹), systemic velocity (−126.8_−1.5^+2.0 km s⁻¹), and shows that it does not show any sign of a significant velocity gradient (d⟨v⟩/dχ = 0.6_−0.4^+0.6 km s⁻¹ arcmin⁻¹, or −0.5/1.9 km s⁻¹ arcmin⁻¹ as 3σ upper limits). We are thus able to confirm the kinematic and metallicity properties of the satellite as well as identify new members for future high-resolution analyses.