APOGEE spectroscopic evidence for chemical anomalies in dwarf galaxies: The case of M~54 and Sagittarius

TL;DR

This study provides spectroscopic evidence of multiple stellar populations and chemical anomalies in the globular cluster M~54 within the Sagittarius dwarf galaxy, highlighting the presence of second-generation stars and tidal debris.

Contribution

First large-scale derivation of [N/Fe] in M~54, revealing multiple populations and chemical anomalies linked to globular cluster evolution in a dwarf galaxy.

Findings

Detection of nitrogen- and aluminum-enriched stars with Mg depletion in M~54

Most stars in M~54 show chemical patterns distinct from Sagittarius field stars

Discovery of a nitrogen-enhanced star likely ejected from M~54

Abstract

We present evidence for globular cluster stellar debris in a dwarf galaxy system (Sagittarius: Sgr) based on an analysis of high-resolution \textit{H}-band spectra from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We add [N/Fe], [Ti/Fe], and [Ni/Fe] abundance ratios to the existing sample of potential members of M~54; this is the first time that [N/Fe] abundances are derived for a large number of stars in M~54. Our study reveals the existence of a significant population of nitrogen- (with a large spread, $\gtrsim1$ dex) and aluminum-enriched stars with moderate Mg depletion in the core of the M~54$+$Sagittarius system, which shares the light element anomalies characteristic of second-generation globular cluster stars (GCs), thus tracing the typical phenomenon of multiple stellar populations seen in other Galactic GCs at similar metallicity, confirming earlier results based on the Na-O anti-correlation. We further show that most of the stars in M~54 exhibit different chemical - patterns evidently not present in Sgr field stars. Furthermore, we report the serendipitous discovery of a nitrogen-enhanced extra-tidal star with GC second-generation-like chemical patterns for which both chemical and kinematic evidence is commensurate with the hypothesis that the star has been ejected from M~54. Our findings support the existence of chemical anomalies associated with likely tidally shredded GCs in dwarf galaxies in the Local Group and motivate future searches for such bonafide stars along other known Milky Way streams.