Chemical composition and constraints on mass loss for globular clusters in dwarf galaxies: WLM and IKN

Abstract

Aims: We determine the metallicities of globular clusters (GCs) in the WLM and IKN dwarf galaxies, using VLT/UVES and Keck/ESI spectroscopy. These measurements are combined with literature data for field stars to constrain GC formation scenarios. For the WLM GC, we also measure detailed abundance ratios for a number of light, α, Fe-peak, and n-capture elements, which are compared with literature data for the Fornax dSph and the Milky Way.
Methods: The abundances are derived by computing synthetic integrated-light model spectra and adjusting the input composition until the best fits to the observed spectra are obtained.
Results: We find low metallicities of [Fe/H] = -2.0 and -2.1 for the WLM GC and the GC IKN-5, respectively. We estimate that 17%-31% of the stars with [Fe/H] ≤ -2 in WLM belong to the GC, and IKN-5 may even contain a similar number of metal-poor stars as the whole of the IKN dwarf itself. While these fractions are much higher than in the Milky Way halo, we have previously found a similarly high ratio of metal-poor GCs to field stars in the Fornax dSph. The overall abundance patterns in the WLM GC are similar to those observed for GCs in the Fornax dSph: the [Ca/Fe] and [Ti/Fe] ratios are super-Solar at about +0.3 dex, while [Mg/Fe] is less elevated than [Ca/Fe] and [Ti/Fe]. The [Na/Fe] ratio is similar to the averaged [Na/Fe] ratios in Milky Way GCs, but higher (by ~2σ) than those of Milky Way halo stars. Iron-peak (Mn, Sc, Cr) and heavy elements (Ba, Y, La) generally follow the trends seen in the Milky Way halo.
Conclusions: The GCs in the WLM and IKN dwarf galaxies resemble those in the Fornax dSph by being significantly more metal-poor than a typical halo GC in the Milky Way and other large galaxies. They are also substantially more metal-poor than the bulk of the field stars in their parent galaxies. It appears that only a small fraction of the Milky Way GC system could have been accreted from galaxies similar to these dwarfs. The relatively high Na abundance in the WLM GC suggests that the [Na/O] anti-correlation is present in this cluster, while the high ratios of metal-poor GCs to field stars in the dwarfs are in tension with GC formation scenarios that require GCs to have lost a very large fraction of their initial mass.

Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme ID 077.B-0354(A), and at the W. M. Keck Observatory, which is operated as a partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration.Tables 1 and 2 are available in electronic form at http://www.aanda.org