The intermediate age star cluster W3 ( age \approx 300 – 500 Myr ) in NGC 7252 is the most luminous star cluster known to date with a dynamical mass estimate of 8 \pm 2 \cdot 10 ^ { 7 } M _ { \odot } .
With an effective radius of about 17.5 pc and a velocity dispersion of 45 kms ^ { -1 } this object may be viewed as one of the recently discovered ultra-compact dwarf galaxies ( UCDs ) .
Its intermediate age , however , precludes an origin as a stripped nucleated dwarf elliptical galaxy .

The galaxy NGC 7252 is a merger remnant from two gas-rich disc galaxies .
Interactions between two gas-rich galaxies lead to bursts of intense star formation .
The age of the interaction and the age of W3 are in good agreement , suggesting that was W3 probably formed in the starburst .

We propose a formation scenario for W3 .
Observations of interacting galaxies reveal regions of strong star formation forming dozens up to hundreds of star cluster in confined regions of up to several hundred parsec in diameter .
The total mass of new stars in these regions can reach 10 ^ { 7 } or even 10 ^ { 8 } M _ { \odot } .
By means of numerical simulations we have shown that the star clusters in these regions merge on short time-scales of a few Myr up to a few hundred Myr .
We apply this scenario to W3 and predict properties which could be observable and the future evolution of this object .

This work lends credence to the notion that at least some of the UCDs may be evolved star cluster complexes formed during early hierarchical mergers .