The Pristine Dwarf-Galaxy survey - II. In-depth observational study of
  the faint Milky Way satellite Sagittarius II
  We present an extensive study of the Sagittarius II (Sgr II) stellar system
using MegaCam g and i photometry, narrow-band, metallicity-sensitive Calcium
H&K doublet photometry, augmented with Keck II/DEIMOS multi-object
spectroscopy. We are able to derive and refine the Sgr II structural and
stellar properties: the colour-magnitude diagram implies Sgr II is old (12.0 +-
0.5 Gyr) and metal-poor. The CaHK photometry confirms the metal-poor nature of
the satellite ([Fe/H]CaHK = -2.32 +- 0.04 dex) and suggests that Sgr II hosts
more than one single stellar population ({\sigma} CaHK = 0.11+0.05-0.03 dex).
From the deep spectroscopic data, the velocity dispersion of the system is
found to be {\sigma}vr = 2.7+1.3-1.0 km s-1 after excluding two potential
binary stars. Using the Ca infrared triplet measured from our highest
signal-to-noise spectra, we are able to confirm the metallicity and dispersion
inferred from the Pristine photometric metallicities: ([Fe/H] = -2.23 +- 0.05
dex, {\sigma}spectro = 0.10+0.06-0.04 dex). Sgr II's metallicity and absolute
magnitude (MV = -5.7 +- 0.1 mag) place the system on the luminosity-metallicity
relation of the Milky Way dwarf galaxies despite its small size. The low, but
resolved metallicity and velocity dispersions paint the picture of a slightly
dark matter-dominated satellite. Furthermore, using the Gaia Data Release 2, we
constrain the orbit of the satellite and find an apocenter of 118.4+28.4-23.7
kpc and a pericenter of 54.8+3.3-6.1 kpc. The orbit of Sgr II is consistent
with the trailing arm of the Sgr stream and indicates that it is possibly a
satellite of the Sgr dSph that was tidally stripped from the dwarf's influence.
