Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 491, Issue 1, Pages 356-377Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stz2854
Keywords
galaxies: dwarf; Local Group
Categories
Funding
- Emmy Noether programme from the Deutsche Forschungsgemeinschaft (DFG)
- French National Research Agency as part of the investments for the future program
- French National Research Agency (ANR) [ANR-18-CE31-0017]
- CNRS/INSU through the Programme National Galaxies et Cosmologie
- CNRS/INSU through the CNRS [PICS07708]
- International Space Science Institute (ISSI), Berne, Switzerland
- Spanish Ministry project MINECO [AYA2017-86389-P]
- Spanish MINECO under the 2013 Ramon y Cajal program MINECO [RYC-2013-14875]
- FONDECYT [3160510]
- W. M. Keck Foundation
- National Aeronautics and Space Administration through the Planetary Science Division of the NASA Science Mission Directorate [NNX08AR22G]
- National Science Foundation [AST-1238877]
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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 (sigma(CaHK)([FeH]) = 0.11(-0.03)(+0.05) 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, sigma(spectro)([Fe/H]) = 0.10(-0.04)(+0.06) dex). The velocity dispersion of the system is found to be sigma(v) = 2.7(-1.0)(+1.3) km s(-1) after excluding two potential binary stars. Sgr II's metallicity and absolute magnitude (M-V = -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(-23.0)(+32.8) M-circle dot L-circle dot(-1)). Furthermore, using the Gaia Data Release 2, we constrain the orbit of the satellite and find an apocentre of 118.4(-23.7)(+28.4) kpc and a pericentre of 54.8(-6.1)(+3.3) 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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