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 broadband $g$ and $i$ photometry and narrow-band metallicity-sensitive CaHK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modeling 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 $\pm 0.5$ Gyr), very metal poor, very faint ($L_V=180_{-72}^{+124}{L}_{\odot }$), and at a distance $d=21.5\pm 0.4$ kpc. The narrow-band, metallicity-sensitive CaHK Pristine photometry confirms this very low metallicity ([Fe/H]$=-2.7\pm 0.1$ 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 ($\ddot{I}{ƒ}_{vr}<5.9$ 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 ($⟨v_r⟩=-342.5_{-1.2}^{+1.1}$ km s${}^{-1}$). We further show that the spectroscopically confirmed members of Dra~II have a mean proper motion of $(\hat{I}{¼}_{\hat{I}}{\pm }^{\ast },\hat{I}{¼}_{\hat{I}}´)=(1.26\pm 0.27,0.94\pm 0.28)$ mas yr${}^{-1}$ in the Gaia DR2 data, which translates to an orbit with a pericenter and an apocenter 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.

Accepted for publication in MNRAS