The pristine dwarf-galaxy survey – III. Revealing the nature of the Milky Way globular cluster Sagittarius II

Abstract

We present a new spectroscopic study of the faint Milky Way satellite Sagittarius II. Using multi-object spectroscopy from the Fibre Large Array Multi Element Spectrograph, we supplement the dataset of Longeard et al. (2020) with 47 newly observed stars, 19 of which are identified as members of the satellite. These additional member stars are used to put tighter constraints on the dynamics and the metallicity properties of the system. We find a low velocity dispersion of $\sigma _\mathrm{v}^\mathrm{SgrII} = 1.7 \pm 0.5$ km s−1, in agreement with the dispersion of Milky Way globular clusters of similar luminosity. We confirm the very metal-poor nature of the satellite ([Fe/H]$_\mathrm{spectro}^\mathrm{SgrII} = -2.23 \pm 0.07$) and find that the metallicity dispersion of Sgr II is not resolved, reaching only 0.20 at the 95 per cent confidence limit. No star with a metallicity below −2.5 is confidently detected. Therefore, despite the unusually large size of the system (r$_h = 35.5 ^{+1.4}_{-1.2}$ pc), we conclude that Sgr II is an old and metal-poor globular cluster of the Milky Way.

Stellar mass segregation as separating classifier between globular clusters and ultra-faint dwarf galaxies

“…We therefore conclude that Sagittarius II is a star cluster. This classification is in agreement with the fact that Longeard et al (2021) find no metallicity spread among the member stars of Sagittarius II.…”

“…Sagittarius II was discovered as a stellar overdensity by Laevens et al (2015) in Pan-STARRS 1 data. Longeard et al (2020) and Longeard et al (2021) obtained stellar radial velocities and metallicity estimates for about 25 member stars of Sagittarius II from which Longeard et al (2021) determined a velocity dispersion of about 1.7 ± 0.5 km/sec. Such a velocity dispersion would be significantly higher than the one expected for a stellar system with the size and mass of Sagittarius II, since, using the N -body models of Baumgardt (2017), we predict a velocity dispersion of only about 0.5 km/sec for Sagittarius II.…”

Star Formation Histories of Ultra-faint Dwarf Galaxies: Environmental Differences between Magellanic and Non-Magellanic Satellites?*

“…We include Sag 2 in our sample, though there is conflicting evidence of it being either a UFD (Longeard et al 2020) or a globular cluster (Mutlu-Pakdil et al 2018;Longeard et al 2021); spectroscopic follow-up observations would be the ultimate confirmation of its true nature.…”

Gaia early DR3 systemic motions of Local Group dwarf galaxies and orbital properties with a massive Large Magellanic Cloud

“…For the MW, we consider satellites those galaxies that have the 16 th quantile of their apocenter within the virial radius of the MW in any of the 3 potentials considered in Section 7, plus the Sgr dSph and the galaxies not assigned to the LMC; Sagittarius II and Crater I are not taken into account, since they are likely globular clusters (e.g. Laevens et al 2014;Voggel et al 2016;Longeard et al 2021). In the generous MW sample, we also add the galaxies for which spectroscopic measurements are not available.…”