Detailed Chemical Composition and Orbit of the Newly Discovered Globular Cluster FSR 1758: Implications for the Accretion of the Sequoia Dwarf Galaxy onto the Milky Way*

Abstract

We present detailed chemical abundances, radial velocity, and orbital parameters for FSR 1758, a recently discovered massive cluster in the direction of the Galactic bulge. High-resolution ( R  ∼ 42,000) spectra were obtained for nine members using the Magellan /Clay telescope instrumented with the MIKE echelle spectrogragh in the wavelength range of ∼4900–8700 Å. Cluster membership was determined using Gaia DR2 proper motions and confirmed with our radial velocity measurements. We find a metallicity of [Fe/H] = −1.58 ± 0.03, consistent with previous photometric estimates, and no significant iron spread. While other studies have suggested that this massive object could be the remnant of a captured dwarf galaxy, our results are consistent with a globular cluster (GC) nature, given its lack of any intrinsic metallicity spread and the Na–O anticorrelation similar to those of other GC. In addition, the small velocity dispersion of 4.9 ± 1.2 km s −1 we find is that typical of a GC. We also confirm a retrograde orbit that appears to be highly eccentric suggesting it is a halo interloper currently in the bulge. We support the hypothesis that FSR 1758 was part of a disrupted dwarf galaxy named Sequoia.