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.ES, KY, and AA gratefully acknowledge funding by the Emmy
Noether programme from the Deutsche Forschungsgemeinschaft
(DFG). This work has been published under the framework of the
IdEx Unistra and benefits from a funding from the state managed
by theFrench National Research Agency as part of the investments
for the future program. NFM, RI, and NL gratefully acknowledge
support from the French National Research Agency (ANR) funded
project ‘Pristine’ (ANR-18-CE31-0017) along with funding from
CNRS/INSU through the Programme National Galaxies et Cosmologie and through the CNRS grant PICS07708. The authors
thank the International Space Science Institute (ISSI), Berne,
Switzerland for providing financial support and meeting facilities
to the international team ‘Pristine’. JIGH acknowledges financial
support from the Spanish Ministry project MINECO AYA2017-
86389-P, and from the Spanish MINECO under the 2013 Ramon y ´
Cajal program MINECO RYC-2013-14875.
BPML gratefully acknowledges support from FONDECYT postdoctoral fellowship No. 316051
