A possible formation scenario for the ultramassive cluster W3 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 time-s cales 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.

[1]  Jean P. Brodie,et al.  Hst Observations of Star Clusters in Ngc 1023: Evidence for Three Cluster Populations? , 2000 .

[2]  P. Kroupa,et al.  SUPERBOX – an efficient code for collisionless galactic dynamics , 2000 .

[3]  B. Elmegreen,et al.  A Universal Formation Mechanism for Open and Globular Clusters in Turbulent Gas , 1997 .

[4]  P. Kroupa,et al.  The formation of ultracompact dwarf galaxies , 2001 .

[5]  William E. Harris,et al.  A Catalog of Parameters for Globular Clusters in the Milky Way , 1996 .

[6]  Denis Foo Kune,et al.  Starburst99: Synthesis Models for Galaxies with Active Star Formation , 1999, astro-ph/9902334.

[7]  Pavel Kroupa,et al.  Evidence for a fundamental stellar upper mass limit from clustered star formation, and some implications therof , 2003, Proceedings of the International Astronomical Union.

[8]  Mario Mateo,et al.  DWARF GALAXIES OF THE LOCAL GROUP , 1998, astro-ph/9810070.

[9]  H Germany,et al.  The central region of the Fornax cluster - III. Dwarf galaxies, globular clusters, and cD halo – are there interrelations? , 1999, astro-ph/9905112.

[10]  Galaxy threshing and the origin of ultra-compact dwarf galaxies in the Fornax cluster , 2003, astro-ph/0308243.

[11]  S. M. Fall,et al.  The Luminosity Function of Young Star Clusters in “the Antennae” Galaxies (NGC 4038/4039) , 1999, astro-ph/9907430.

[12]  P. Kroupa.  On the variation of the initial mass function , 2000, astro-ph/0009005.

[13]  M. Fellhauer,et al.  The Possible Origin of the Faint Fuzzy Star Clusters in NGC 1023 , 2002, astro-ph/0207046.

[14]  Ultracompact Dwarf Galaxies in the Fornax Cluster , 2001, astro-ph/0106377.

[15]  M. Hilker,et al.  Fornax compact object survey FCOS: On the nature of Ultra Compact Dwarf galaxies , 2004, astro-ph/0401610.

[16]  S. M. Fall,et al.  The Mass Function of Young Star Clusters in the “Antennae” Galaxies , 1999, The Astrophysical journal.

[17]  P. Kroupa,et al.  Merging Timescales and Merger Rates of Star Clusters in Dense Star Cluster Complexes , 2002 .

[18]  L. E. Davis,et al.  CCD surface photometry of galaxies with dynamical data. II. UBR photometry of 39 elliptical galaxies , 1990 .

[19]  Stuart L. Shapiro,et al.  Random Gravitational Encounters and the Evolution of Spherical Systems. III. Halo , 1971 .