Spectroscopic metallicities for Fornax ultracompact dwarf galaxies, globular clusters, and nucleated dwarf elliptical galaxies

Various formation channels for the puzzling ultracompact dwarf galaxies (UCDs) have been proposed in the last few years. To betterjudge some of the competing scenarios, we present spectroscopic [Fe/H] estimates for a sample of 26 compact objects in the central region of the Fornax Cluster, covering the magnitude range of UCDs and bright globular clusters (18 mag < V < 21 mag or -13.4 mag < M_V < -10.4 mag). We find a break in the metallicity distribution of compact objects at M_V ≃ -11 mag (≃3 × 10⁶ M⊙): for M_V < -11 mag the mean metallicity is [Fe/H] = -0.62 ± 0.05 dex, 0.56 ± 0.15 dex higher than the value of -1.18 ± 0.15 dex found for M_V > -11 mag. This metallicity break is accompanied by a change in the size-luminosity relation for compact objects, as deduced from Hubble Space Telescope imaging: for M_V < -11 mag, r_h scales with luminosity, while for M_V > -11 mag, r_h is almost luminosity-independent. In our study we therefore assume a limiting absolute magnitude of M_V = -11 mag between UCDs and globular clusters. The mean metallicity of five Fornax nucleated dwarf elliptical galaxy (dE,N) nuclei included in our study is about 0.8 dex lower than that of the UCDs, a difference significant at the 4.5 σ level. This difference is marginally higher than expected from a comparison of their (V - I) colors, indicating that UCDs are younger than or at most coeval to dE,N nuclei. Because of the large metallicity discrepancy between UCDs and nuclei, we disfavor the hypothesis that most of the Fornax UCDs are the remnant nuclei of tidally stripped dE,Ns. Our metallicity estimates for UCDs are closer to but slightly below those derived for young massive clusters (YMCs) of comparable masses. We therefore favor a scenario in which most UCDs in Fornax are successors of merged YMCs produced in the course of violent galaxy-galaxy mergers. It is noted that, in contrast, the properties of Virgo UCDs are more consistent with the stripping scenario, suggesting that different UCD formation channels may dominate in either cluster.