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arXiv · 1902.02780

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

Nicolas Longeard·Nicolas Martin·Else Starkenburg·Rodrigo A. Ibata·Michelle L. M. Collins·Benjamin P. M. Laevens·Dougal Mackey·R. Michael Rich·David S. Aguado·Anke Arentsen·Pascale Jablonka·Jonay I. Gonzalez Hernandez·Julio F. Navarro·Ruben Sanchez-Janssen
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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, 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.

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Nicolas Longeard, Nicolas Martin, Else Starkenburg, Rodrigo A. Ibata, Michelle L. M. Collins, Benjamin P. M. Laevens, Dougal Mackey, R. Michael Rich, David S. Aguado, Anke Arentsen, Pascale Jablonka, Jonay I. Gonzalez Hernandez, Julio F. Navarro, Ruben Sanchez-Janssen. 2019-02-07. The Pristine Dwarf-Galaxy survey - II. In-depth observational study of the faint Milky Way satellite Sagittarius II. https://doi.org/10.1093/mnras/stz2854

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