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 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 ^{+124}_{-72} 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 ($\sigma_{vr}<5.9$ 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 ($\langle v_{r}\rangle =
-342.5^{+1.1}_{-1.2} $ km s$^{-1}$). We further show that the spectroscopically
confirmed members of Dra~II have a mean proper motion of
$(\mu_\alpha^*,\mu_\delta)=(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 $21.3 ^{+0.7}_{-1.0}$ and $153.8 ^{+56.7}_{-34.7}$ 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.
