Ultra compact objects in the Fornax cluster of galaxies: Globular clusters or dwarf galaxies?

Abstract

The relation between the Ultra Compact Objects (hereafter UCOs) recently discovered in the Fornax cluster (Drinkwater et al. 2000a; Hilker et al. 1999) and the brightest globular clusters associated with the central galaxy NGC 1399 has been investigated. The question was adressed whether the UCOs constitute a distinct population of objects not linked to globular clusters or whether there is a smooth transition between both populations. Therefore, a spectroscopic survey on compact objects in the central region of the Fornax cluster was carried out with the 2.5 m du Pont telescope (LCO). UCOs and the bright NGC 1399 globular clusters with similar brightness were inspected. 12 GCs from the bright end of the globular cluster luminosity function have been identified as Fornax members. Eight are new members, four were known as members from before. Their magnitude distribution supports a smooth transition between the faint UCOs and the bright globular clusters. There is no evidence for a magnitude gap between both populations. However, the brightest UCO clearly stands out; it is too bright and too large to be accounted for by globular clusters. For the only UCO included in our survey, a relatively high metallicity of [Fe/H] ≃ -0.5 dex is measured.