2004A&A...416..467M

We have determined the dynamical mass of the most luminous stellar cluster known to date, i.e. object W3 in the merger remnant galaxy NGC 7252. The dynamical mass is estimated from the velocity dispersion measured with the high-resolution spectrograph UVES on VLT. Our result is the astonishingly high velocity dispersion of σ=45±5km/s. Combined with the large cluster size R_eff=17.5±1.8pc, this translates into a dynamical virial mass for W3 of (8±2)x10⁷M_☉. This mass is in excellent agreement with the value (∼7.2x10⁷M_☉) we previously estimated from the cluster luminosity (M_V=-16.2) by means of stellar M/L ratios predicted by Simple Stellar Population models (with a Salpeter IMF) and confirms the heavy-weight nature of this object. This results points out that the NGC 7252-type of mergers are able to form stellar systems with masses up to ∼10⁸M_☉. We find that W3, when evolved to ∼10Gyr, lies far from the typical Milky Way globular clusters, but appears to be also separated from ωCen in the Milky Way and G1 in M31, the most massive old stellar clusters of the Local Group, because it is too extended for a given mass, and from dwarf elliptical galaxies because it is much more compact for its mass. Instead the aged W3 is amazingly close to the compact objects named ultracompact dwarf galaxies (UCDGs) found in the Fornax cluster (Hilker et al., 1999A&AS..134...75H; Drinkwater et al., 2000PASA...17..227D), and to a miniature version of the compact elliptical M32. These objects start populating a previously deserted region of the fundamental plane.