Various formation channels for the puzzling ultra-compact dwarf galaxies ( UCDs ) have been proposed in the last few years .
To better judge on 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 < V < 21 or -13.4 < M _ { V } < -10.4 mag ) .
We find a break in the metallicity distribution of compact objects at M _ { V } \simeq - 11 mag ( \simeq 3 \times 10 ^ { 6 } M _ { \sun } ) : for M _ { V } < -11 mag the mean metallicity is [ Fe/H ] = - 0.62 \pm 0.05 dex , 0.56 \pm 0.15 dex higher than the value of - 1.18 \pm 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 HST-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 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 \sigma 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 where most UCDs in Fornax are successors of merged YMCs produced in the course of violent galaxy-galaxy mergers .
It is noted that in contrast to that , the properties of Virgo UCDs are more consistent with the stripping scenario , suggesting that different UCD formation channels may dominate in either cluster .