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The dynamical mass of the young cluster W3 in NGC 7252. Heavy-weight globular cluster or ultra compact dwarf galaxy?
We have determined the dynamical mass of the most luminous stellarcluster known to date, i.e. object W3 in the merger remnant galaxy NGC7252. The dynamical mass is estimated from the velocity dispersionmeasured with the high-resolution spectrograph UVES on VLT. Our resultis the astonishingly high velocity dispersion of σ=45 ± 5km s-1. Combined with the large cluster sizeReff=17.5 ± 1.8 pc, this translates into a dynamicalvirial mass for W3 of (8 ± 2) × 107Mȯ. This mass is in excellent agreement with the value(7.2× 107 Mȯ) we previouslyestimated from the cluster luminosity (MV=-16.2) by means ofstellar M/L ratios predicted by Simple Stellar Population models (with aSalpeter IMF) and confirms the heavy-weight nature of this object. Thisresults points out that the NGC 7252-type of mergers are able to formstellar systems with masses up to 108Mȯ. We find that W3, when evolved to 10 Gyr, liesfar from the typical Milky Way globular clusters, but appears to be alsoseparated from ωCen in the Milky Way and G1 in M 31, the mostmassive old stellar clusters of the Local Group, because it is tooextended for a given mass, and from dwarf elliptical galaxies because itis much more compact for its mass. Instead the aged W3 is amazinglyclose to the compact objects named ultracompact dwarf galaxies (UCDGs)found in the Fornax cluster (Hilker et al. \cite{Hilker99}; Drinkwateret al. \cite{Drinkwater00}), and to a miniature version of the compactelliptical M 32. These objects start populating a previously desertedregion of the fundamental plane.Based on observations collected at the European Southern Observatory,ID: 67.D-0205(A).
| Stellar population samples at the galactic poles. III - UBVRI observations of proper motion stars near the south pole and the luminosity laws for the halo and old disk populations
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1976ApJS...30..351E&db_key=AST
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Observation and Astrometry data
| Constellation: | Sculpteur |
| Right ascension: | 01h23m22.43s |
| Declination: | -27°12'52.8" |
| Apparent magnitude: | 9.742 |
| Proper motion RA: | 114.7 |
| Proper motion Dec: | -73.5 |
| B-T magnitude: | 10.758 |
| V-T magnitude: | 9.826 |
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