Detailed abundance analysis of globular clusters in the Local Group

Abstract

Context. Globular clusters (GCs) are emerging as powerful tracers of the chemical composition of extragalactic stellar populations. Aims. We present new abundance measurements for eleven GCs in the Local Group galaxies NGC 147, NGC 6822, and Messier 33. These are combined with previously published observations of four GCs in the Fornax and WLM galaxies. Methods. The abundances were determined from analysis of integrated-light spectra, obtained with the HIRES spectrograph on the Keck I telescope and with UVES on the VLT. We used our analysis technique that has been developed for this purpose and tested on Milky Way GCs. Results. We find that the clusters with [Fe/H] < −1.5 are all α-enhanced at about the same level as Milky Way GCs. Their Na abundances are also generally enhanced relative to Milky Way halo stars, suggesting that these extragalactic GCs resemble their Milky Way counterparts in containing significant fractions of Na-rich stars. For [Fe/H] > −1.5, the GCs in M33 are also α-enhanced, while the GCs that belong to dwarfs (NGC 6822 SC7 and Fornax 4) have closer to Solar-scaled α-element abundances. The abundance patterns in SC7 are remarkably similar to those in the Galactic GC Ruprecht 106, including significantly sub-solar [Na/Fe] and [Ni/Fe] ratios. In NGC 147, the GCs with [Fe/H] < −2.0 account for about 6% of the total luminosity of stars in the same metallicity range, a lower fraction than those previously found in the Fornax and WLM galaxies, but substantially higher than in the Milky Way halo. Conclusions. At low metallicities, the abundance patterns suggest that GCs in the Milky Way, dwarf galaxies, and M33 experienced similar enrichment histories and/or processes. At higher metallicities, the lower levels of α-enhancement in the GCs found in dwarf galaxies resemble the abundance patterns observed in field stars in nearby dwarfs. Constraining the presence of multiple populations in the GCs is complicated by the lack of information about detailed abundances in field stars of the corresponding metallicities. We suggest that correlations such as [Na/Fe] vs. [Ni/Fe] may prove useful for this purpose if an accuracy of ∼ 0.1 dex or better can be reached for the integrated-light measurements.

Observational constraints on the origin of the elements

“…The data from Galactic and extragalactic globular clusters (Larsen et al 2017(Larsen et al , 2018 also follow the [Mn/Fe] trends seen in data of field stars. This is interesting because globular clusters probe different star formation environments and ex-situ origin has been proposed for a substantial fraction, namely one quarter, of the Galactic GC systems (Pritzl et al 2005;Forbes & Bridges 2010).…”

“…In addition, we include [Mn/Fe] abundances for seven Galactic globular clusters, obtained from integrated-light observations with UVES at VLT (Larsen et al 2017) and 15 extragalactic GCs observed with UVES and with HIRES on the A38, page 2 of 7 Keck I telescope (Larsen et al 2018). These abundance measurements were made by fitting custom-computed simple stellar population models to the integrated-light spectra, adjusting the input abundances in the models until the best fits to the data were obtained.…”

Globular Clusters and the Halos of Dwarf Galaxies

“…Interestingly, these clusters both have approximately Solar-scaled α-element abundances, in agreement with the trends seen in field stars in dwarf galaxies [25]. Cluster SC7 also exhibits a number of other peculiarities, including a significantly sub-Solar [Sc/Fe] ratio and low [Ni/Fe] and [Na/Fe] ratios [26]-indeed, its detailed chemical abundance patterns closely match those in the galactic GC Ruprecht 106, for which an accretion origin has been suggested [27]. A few GCs in the M31 with similar abundance patterns may likewise be accreted [23,24].…”

What are the implications of stellar chemical abundances in dwarf galaxies?

“…Previous work (Suda et al 2017) discussed many aspects of stellar archaeology such as chemical evolution, astrometry, nucleosynthesis in stars, and comparison with the stars in the Galactic globular clusters. Abundance data can be applied to other events and phenomena; chemodynamical evolution of galaxies (Hirai et al 2018), comparisons with stellar yields from rotating first stars (Takahashi et al 2018), abundances in globular clusters in the local group (Larsen et al 2018) comparisons with the simulations of Λ CDM universe (Yves & Pascale), and so on.…”