Pristine dwarf galaxy survey - I. A detailed photometric and spectroscopic study of the very metal-poor Draco II satellite

We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deepCFHT/MegaCam broad-band g and i photometry and narrow-band metallicity-sensitive CaHKobservations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modelling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour–magnitude space implies Dra II is old (13.5± 0.5 Gyr), very metal-poor, very faint (L_V = 180+124^-72 L_⊙), and at a distance d = 21.5 ± 0.4 kpc. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H] = −2.7 ± 0.1 dex). Even though our study benefits from a doubling of thespectroscopic sample size compared to previous investigations, the velocity dispersion of thesystem is still only marginally resolved (σ_vr < 5.9 km s⁻¹ at the 95 per cent confidence level)and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity ( ⟨vr⟩ =−342.5_−1.2^+1.1  km s⁻¹). We further show that the spectroscopically confirmed membersof Dra II have a mean proper motion of (μ^∗_α,μδ ) = (1.26 ± 0.27, 0.94 ± 0.28) mas/yr in theGaia DR2 data, which translates to an orbit with a pericentre and an apocentre of 21.3_−1.0^+0.7  and 153.8_−34.7^+56.7  kpc, respectively. Taken altogether, these properties favour the scenario of Dra II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario.