S316p.106 — From a star cluster ensemble to its formation history

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Aug 10th at 6:00 PM until 7:30 PM

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Author(s): Christine Schulz1, Michael Hilker1

Institution(s): 1. European Southern Observatory (ESO)

Ultra-compact dwarf galaxies (UCDs) populate the high mass end of compact stellar systems. In fact, they share many properties with massive globular clusters (GCs) and are found in similar environments. In the Fornax galaxy cluster, the majority of UCDs are compatible with being the result of star cluster formation processes - as well as GCs are - while the rest of the UCDs probably formed by stripping off the stellar envelope of nucleated dwarf galaxies in the tidal field of the galaxy cluster. After excluding UCDs with dwarf galaxy origin we use the remaining GC/UCD sample as tracers of star formation events - in which the GCs/UCDs probably formed - during the assembly of the galaxies in the center of the galaxy cluster.
The present day GC/UCD sample in Fornax is interpreted to be a superposition of individual star cluster populations formed in different star formation events in the past. It is assumed that every star cluster population forms during a formation epoch of length dt at a constant star-formation rate (SFR). The mass distribution of such a population is described by the embedded cluster mass function (ECMF) being a pure power law extending to an upper limit Mmax. Here, the observational finding is used that higher SFRs are connected to larger Mmax which is known as the SFR-Mmax relation.
Only the high-mass end of the observed GC/UCD mass function is considered since high-mass star clusters are expected to suffer less from the tidal field of the host galaxy cluster, i.e. it is assumed that all high mass GCs/UCDs survived since their formation. Then, the considered part of the GC/UCD mass function is corrected for mass loss as well as for objects which did not form in a star cluster formation process. Finally, the GC/UCD mass function is iteratively decomposed into individual populations which are assumed to be distributed according to the ECMF with an individual upper limit Mmax. From the upper mass limit, Mmax, of each involved ECMF the required SFR is calculated. The combination of all SFRs reveals under which conditions the whole present GC/UCD sample formed, which gives a hint of how the core of the Fornax galaxy cluster assembled.