The Pristine Dwarf-Galaxy survey – II. In-depth observational study of the faint Milky Way satellite Sagittarius II
Résumé
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 (|σCaHK[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+1.3−1.0kms−1| after excluding two potential binary stars. Sgr II’s metallicity and absolute magnitude ( = −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+28.4−23.7kpc| and a pericentre of |54.8+3.3−6.1kpc|. 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.
Domaines
Astrophysique [astro-ph]Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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