A Possible Formation Scenario for the Ultra - Massive Cluster W 3 in NGC 7252

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 · 10 7 M ⊙. 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 ⊙. By means of numerical simulations we have shown that the star clusters in these regions merge on short timescales 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.