CAPOS: the bulge Cluster APOgee Survey II. The Intriguing "Sequoia" Globular Cluster FSR 1758

TL;DR

This study confirms FSR 1758 as a globular cluster with multiple stellar populations, refines its metallicity, and suggests its association with the Gaia-Enceladus-Sausage event rather than the Sequoia dwarf galaxy.

Contribution

First spectroscopic evidence of multiple populations in FSR 1758 and clarification of its origin and dynamical association using high-resolution near-infrared spectra.

Findings

Confirmed multiple populations via N-C anti-correlation and Al-N correlation.

Determined metallicity between -1.43 and -1.36, slightly more metal-rich than previous reports.

Dynamical analysis links FSR 1758 to Gaia-Enceladus-Sausage, not the Sequoia galaxy.

Abstract

We present results from a study of fifteen red giant members of the intermediate-metallicity globular cluster (GC) FSR 1758 using high-resolution near-infrared spectra collected with the Apache Point Observatory Galactic Evolution Experiment II survey (APOGEE-2), obtained as part of CAPOS (the bulge Cluster APOgee Survey). Since its very recent discovery as a massive GC in the bulge region, evoking the name Sequoia, this has been an intriguing object with a highly debated origin, and initially led to the suggestion of a purported progenitor dwarf galaxy of the same name. In this work, we use new spectroscopic and astrometric data to provide additional clues to the nature of FSR 1758. Our study confirms the GC nature of FSR 1758, and as such we report for the first time the existence of the characteristic N-C anti-correlation and Al-N correlation, revealing the existence of the multiple-population phenomenon, similar to that observed in virtually all GCs. Furthermore, the presence of a population with strongly enriched aluminium makes it unlikely FSR 1758 is the remnant nucleus of a dwarf galaxy, as Al-enhanced stars are uncommon in dwarf galaxies. We find that FSR 1758 is slightly more metal rich than previously reported in the literature, with a mean metallicity [Fe/H] between $-1.43$ to $-1.36$ (depending on the adopted atmospheric parameters), and with a scatter within observational error, again pointing to its GC nature. Overall, the $\alpha$-enrichment ($\gtrsim+0.3$ dex), Fe-peak (Fe, Ni), light- (C, N), and odd-Z (Al) elements follow the trend of intermediate-metallicity GCs. ... A new examination of its dynamical properties with the \texttt{GravPot16} model favors an association with the Gaia-Enceladus-Sausage accretion event. Thus, paradoxically, the cluster that gave rise to the name of the Sequoia dwarf galaxy does not appear to belong to this specific merging event.