The chemical composition of globular clusters in the Local Group

Abstract

We present detailed chemical abundance measurements for 45 globular clusters (GCs) associated with galaxies in (and, in one case, beyond) the Local Group. The measurements are based on new high-resolution integrated-light spectra of GCs in the galaxies NGC 185, NGC 205, M 31, M 33, and NGC 2403, combined with reanalysis of previously published observations of GCs in the Fornax dSph, WLM, NGC 147, NGC 6822, and the Milky Way. The GCs cover the range −2.8 < [Fe/H] < −0.1 and we determined abundances for Fe, Na, Mg, Si, Ca, Sc, Ti, Cr, Mn, Ni, Cu, Zn, Zr, Ba, and Eu. Corrections for non local thermodynamic equilibrium effects are included for Na, Mg, Ca, Ti, Mn, Fe, Ni, and Ba, building on a recently developed procedure. For several of the galaxies, our measurements provide the first quantitative constraints on the detailed composition of their metal-poor stellar populations. Overall, the GCs in different galaxies exhibit remarkably uniform abundance patterns of the α, iron-peak, and neutron-capture elements, with a dispersion of less than 0.1 dex in [α/Fe] for the full sample. There is a hint that GCs in dwarf galaxies are slightly less α-enhanced (by ∼0.04 dex on average) than those in larger galaxies. One GC in M 33 (HM33-B) resembles the most metal-rich GCs in the Fornax dSph (Fornax 4) and NGC 6822 (SC7) by having α-element abundances closer to scaled-solar values, possibly hinting at an accretion origin. A principal components analysis shows that the α-element abundances strongly correlate with those of Na, Sc, Ni, and Zn. Several GCs with [Fe/H] < −1.5 are deficient in Mg compared to other α-elements. We find no GCs with strongly enhanced r-process abundances as reported for metal-poor stars in some ultra-faint dwarfs and the Magellanic Clouds. The similarity of the abundance patterns for metal-poor GCs in different environments points to similar early enrichment histories and only allow for minor variations in the initial mass function.

Introducing EMP-Pathfinder: modelling the simultaneous formation and evolution of stellar clusters in their host galaxies

“…The relative abundance of [Mg/Fe] decreases towards higher metallicities in both populations. Comparing to the high-resolution abundances from Larsen et al (2022), we find that our simulated abundances are consistent with the observational data for metallicities [Fe/H] < −1.5 dex. However, at higher metallicities, the [Mg/Fe] ratio drops to [Mg/Fe] ∼ −0.15, whereas in the Milky Way and M31 it remains constant at [Mg/Fe] ∼ 0.25.…”

“…(Bottom-right panel) [Mg/Fe] vs [Fe/H] abundances of old stellar clusters in two representative simulated galaxies. We include the Mg and Fe abundances for GCs in the MW and M31 (Larsen et al 2022) as coloured stars, as well as the abundances for individual stars in Galactic GCs (Roediger et al 2014) as red circles.…”

N-enhancement in GN-z11: First evidence for supermassive stars nucleosynthesis in proto-globular clusters-like conditions at high redshift?

“…No signature of high He-enrichment and of other nucleosynthesis processes such as triple−α, s-process, and explosive nucleosynthesis was found in the vast majority of the GCs, except in the most massive ones that might be the remnants of dwarf galaxy nuclei (Marino et al 2021). The same chemical peculiarities were found in all the GCs of the Milky Way and of the Local Group where they have been looked for (Milone et al 2017;Larsen et al 2022), as well as in extragalactic massive star clusters of intermediate-age down to ∼2 Gyr (Martocchia et al 2018;Bastian et al 2019), pointing to similar early enrichment histories. However, they were never observed in less massive open clusters, implying a minimum cluster mass ( a few 10 4 M , M V -5; Bragaglia et al 2017) for their formation.…”

The Pristine Inner Galaxy Survey (PIGS) – V. A chemo-dynamical investigation of the early assembly of the Milky Way with the most metal-poor stars in the bulge

“…The bulge compilation is from the high-resolution optical spectral analyses by Howes et al ( 2014Howes et al ( , 2015Howes et al ( , 2016, Koch et al ( 2016 ), Reggiani et al ( 2020 ), andLucey et al ( 2022 ). A compilation of stars with high-resolution spectral analyses in globular clusters was collected from Pancino et al ( 2017), Pritzl, Venn & Irwin ( 2005, Larsen et al ( 2022 ), andMartin et al ( 2022 ), and for stars in a selection of dwarf galaxies, including Hercules (Koch et al 2008(Koch et al , 2013Fran c ¸ois et al 2016 ) and Segue 1 (Frebel, Simon & Kirby 2014 ).…”