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 \sigma = 45 \pm 5 ~ { } { km / s } .
Combined with the large cluster size R _ { eff } = 17.5 \pm 1.8 ~ { } { pc } , this translates into a dynamical virial mass for W3 of ( 8 \pm 2 ) 10 ^ { 7 } M _ { \odot } .
This mass is in excellent agreement with the value ( \sim 7.2 \cdot 10 ^ { 7 } ~ { } M _ { \odot } ) 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 \sim~ { } 10 ^ { 8 } ~ { } M _ { \odot } .
We find that W3 , when evolved to \sim 10 Gyr , lies far from the typical Milky Way globular clusters , but appears to be also separated from \omega Cen 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 .
1999 ; Drinkwater et al .
2000 ) , and to a miniature version of the compact elliptical M32 .
These objects start populating a previously deserted region of the fundamental plane .