Abundance ties: Nephele and the globular cluster population accreted with ω Cen
Authors: Pagnini G. , Di Matteo P. , Haywood M. , Mastrobuono-Battisti A. , Renaud F. , Mondelin M. , Agertz O. , Bianchini P. , Casamiquela L. , Khoperskov S. , Ryde N. 
Published: December 2024

Context. The peculiar Galactic globular cluster ω Centauri (NGC 5139) has drawn attention for its unique features, such as an unusually high stellar mass compared to other Galactic globular clusters and a broad distribution of chemical elements. These features have led to the hypothesis that ω Centauri might be the nuclear remnant of an ancient dwarf galaxy accreted by the Milky Way, potentially bringing along its own globular cluster system.

Aims. In this work, we adopt an innovative approach by examining the individual chemical abundances of Galactic globular clusters to identify shared patterns with ω Centauri.

Methods. Applying Gaussian mixture models to globular cluster stars, whose membership is based on the analysis of the Gaia EDR3 release, and whose chemical abundances have been obtained from APOGEE DR17, we depart from traditional kinematic-based procedures and search for globular clusters that are chemically compatible with ω Centauri in an eight-dimensional space defined by [Fe/H], α-elements such as [Mg/Fe], [Si/Fe], and [Ca/Fe], light+odd-Z elements such as [C/Fe], [Al/Fe], and [K/Fe], and an iron-peak element as [Mn/Fe]. With this approach, clusters that are chemically compatible with ω Centauri are clusters whose chemical patterns are contained in the abundance domain defined by ω Centauri stars.

Results. Our analysis leads to the identification of six globular clusters – NGC 6752, NGC 6656, NGC 6809, NGC 6273, NGC 6205, and NGC 6254 – that exhibit strong chemical similarities with ω Centauri, and that have metallicities that coincide with those of the two known peaks (primary and secondary) of ω Centauri’s metallicity distribution. They all exhibit non-null intrinsic [Fe/H] dispersions, ranging between 0.07 and 0.12 dex, unless the ASPCAP uncertainties had been severely underestimated, and three of them have statistically significant skewed [Fe/H] distributions. Furthermore, the chemical patterns of these clusters lead to the exclusion that they were formed in progenitor galaxies with chemical enrichment histories similar to those of the Large and Small Magellanic Clouds, Sagittarius, and Fornax. Once placed in kinematic spaces such as the energy – angular momentum plane, these clusters result scatter across an extended region, which is predicted by N-body simulations if their common progenitor was sufficiently massive compared to the Milky Way.

Conclusions. Our novel approach suggests a common origin for NGC 6752, NGC 6656, NGC 6809, NGC 6273, NGC 6205, NGC 6254, and ω Centauri, indicating that Nephele, as we propose to call the progenitor in which all these clusters formed, played a substantial role in the Galaxy’s history. The finding that a set of globular clusters can be associated with ω Centauri reinforces the hypothesis that this system is the remnant of a galaxy, and not simply an unusual globular cluster. This study also shows that the spectroscopic data at our disposal have reached the quality needed to compare chemical patterns of stellar systems, to reveal their common origins or exclude their association with specific progenitor galaxies.