The Pristine Dwarf-Galaxy survey - II. In-depth observational study of the faint Milky Way satellite Sagittarius II

Abstract

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 and Keck II/DEIMOS multiobject spectroscopy. We derive and refine the Sgr II structural and stellar properties inferred at the time of its discovery. 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 (σ CaHK [FeH] = 0.11+0.05 −0.03 dex). Using the Ca infrared triplet measured from our highest signal-to-noise spectra, we confirm the metallicity and dispersion inferred from the Pristine photometric metallicities ([Fe/H]spectro = −2.23 ± 0.05 dex, σspectro [Fe/H] = 0.10+0.06 −0.04 dex). The velocity dispersion of the system is found to be σv = 2.7+1.3 −1.0 km s−1 after excluding two potential binary stars. 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 (M/L = 23.0+32.8 −23.0 M L−1 ). Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocentre of 118.4+28.4 −23.7 kpc and a pericentre 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.