Constraints on mass loss and self-enrichment scenarios for the globular clusters of the Fornax dSph^⋆

¹ Department of Astrophysics / IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: S.larsen@astro.ru.nl
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
⁴ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA

Received: 26 June 2012
Accepted: 19 July 2012

Abstract

Recently, high-dispersion spectroscopy has demonstrated conclusively that four of the five globular clusters (GCs) in the Fornax dwarf spheroidal galaxy are very metal-poor with  [Fe/H]  <  −2. The remaining cluster, Fornax 4, has  [Fe/H]  =  −1.4. This is in stark contrast to the field star metallicity distribution which shows a broad peak around  [Fe/H]  ≈  −1 with only a few percent of the stars having  [Fe/H]  <  −2. If we only consider stars and clusters with  [Fe/H]  <  −2 we thus find an extremely high GC specific frequency, S_N ≈ 400, implying by far the highest ratio of GCs to field stars known anywhere. We estimate that about 1/5–1/4 of all stars in the Fornax dSph with  [Fe/H]  <  −2 belong to the four most metal-poor GCs. These GCs could, therefore, at most have been a factor of 4–5 more massive initially. Yet, the Fornax GCs appear to share the same anomalous chemical abundance patterns known from Milky Way GCs, commonly attributed to the presence of multiple stellar generations within the clusters. The extreme ratio of metal-poor GC- versus field stars in the Fornax dSph is difficult to reconcile with scenarios for self-enrichment and early evolution of GCs in which a large fraction (90%–95%) of the first-generation stars have been lost. It also suggests that the GCs may not have formed as part of a larger population of now disrupted clusters with an initial power-law mass distribution. The Fornax dSph may be a rosetta stone for constraining theories of the formation, self-enrichment and early dynamical evolution of star clusters.