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 \pm 0.5 ) Gyr and metal-poor .
The CaHK photometry confirms the metal-poor nature of the satellite ( [ Fe/H ] _ { \mathrm { CaHK } } = -2.32 \pm 0.04 dex ) and suggests that Sgr II hosts more than one single stellar population ( \sigma _ { \mathrm { [ FeH ] } } ^ { \mathrm { 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 ] _ { \mathrm { spectro } } = -2.23 \pm 0.05 dex , \sigma _ { \mathrm { [ Fe / H ] } } ^ { \mathrm { spectro } } = 0.10 ^ { +0.06 } _ { -0.04 } dex ) .
Sgr II ’ s metallicity and absolute magnitude ( M _ { V } = -5.7 \pm 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 indicate that it is possibly a satellite of the Sgr dSph that was tidally stripped from the dwarf ’ s influence .