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

Abstract

We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broad-band g and i photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-upKeck II/DEIMOSmulti-object spectroscopy. Forwardmodelling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour-magnitude space impliesDra II is old (13.5±0.5Gyr), very metal-poor, very faint (LV = 180-72+124 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 the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved (σvr < 5.9km 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 members of Dra II have a mean proper motion of (μ*α,μδ) = (1.26 ± 0.27, 0.94 ± 0.28) mas/yr in the Gaia 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.