Fornax compact object survey FCOS: On the nature of Ultra Compact Dwarf galaxies

The results of the Fornax Compact Object Survey (FCOS) are presented, complementing the first part of the FCOS described in Mieske et al. (2002). The FCOS aims at investigating the nature of the Ultra Compact Dwarf galaxies (UCDs) recently discovered in the center of the Fornax cluster (Drinkwater et al. 2000a). 280 unresolved objects with colours 0 < (V −I) < 1.5 mag in the magnitude space covering UCDs and bright globular clusters (18 < V < 21 mag) were observed spectroscopically. 54 new Fornax members were found, plus five of the seven already known UCDs. Their distribution in radial velocity, colour, magnitude and space was investigated. It is found that bright compact objects (V < 20 or MV < −11.4 mag), including the UCDs, have a higher mean velocity than faint compact objects (V > 20 mag) at 96% confidence. The mean velocity of the bright compact objects is consistent with that of the dwarf galaxy population in Fornax, but inconsistent with that of NGC 1399’s globular cluster system at 93.5% confidence. The compact objects follow a colour magnitude relation with a slope very similar to that of normal dEs, but shifted about 0.2 mag redwards. The magnitude distribution of compact objects shows a fluent transition between UCDs and GCs. A slight overpopulation of 8 ± 4 objects for V < 20 mag is detected with respect to the extrapolation of NGC 1399’s GC luminosity function. The spatial distribution of bright compact objects is in comparison to the faint ones more extended at 88% confidence. All our findings are consistent with the threshing scenario (Bekki et al. 2003), suggesting that a substantial fraction of compact Fornax members brighter than V ≃ 20 mag could be created by threshing dE,Ns. Fainter than V ≃ 20 mag, the majority of the objects seem to be genuine GCs. Our results are also consistent with merged stellar super-clusters (Fellhauer & Kroupa 2002) as an alternative explanation for the bright compact objects.