  The intermediate age star cluster W3 (age ~ 300-500 Myr) in NGC 7252 is the
most luminous star cluster known to date with a dynamical mass estimate of 8
+/- 2 x 10^7 M_sun. With an effective radius of about 17.5 pc and a velocity
dispersion of 45 km/s 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_sun. 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.
