Mass distributions of star clusters for different star formation histories in a galaxy cluster environment

摘要

Clusters of galaxies usually contain rich populations of globular clusters (GCs). We investigate how different star formation histories (SFHs) shape the final mass distribution of star clusters. We assumed that every star cluster population forms during a formation epoch of length delta t at a constant star-formation rate (SFR). The mass distribution of such a population is described by the embedded cluster mass function (ECMF), which is a pure power law extending to an upper limit M-max. Since the SFR determines M-max, the ECMF implicitly depends on the SFR. Starting with different SFHs, the time-evolution of the SFR, each SFH is divided into formation epochs of length delta t at different SFRs. The requested mass function arises from the superposition of the star clusters of all formation epochs. An improved optimal sampling technique is introduced that allows generating number and mass distributions, both of which accurately agree with the ECMF. Moreover, for each SFH the distribution function of all involved SFRs, F(SFR), is computed. For monotonically decreasing SFHs, we found that F(SFR) always follows a power law. With F(SFR), we developed the theory of the integrated galactic embedded cluster mass function (IGECMF). The latter describes the distribution function of birth stellar masses of star clusters that accumulated over a formation episode much longer than delta t. The IGECMF indeed reproduces the mass distribution of star clusters created according to the superposition principle. Interestingly, all considered SFHs lead to a turn-down with increasing star cluster mass in their respective IGECMFs in a similar way as is observed for GC systems in different galaxy clusters, which offers the possibility of determining the conditions under which a GC system was assembled. Although assuming a pure power-law ECMF, a Schechter-like IGECMF emerges from the superposition principle. In the past decade, a turn-down at the high-mass end has been observed in the cluster initial mass function. This turn-down can be explained naturally if the observed star cluster ensembles are superpositions of several individual star cluster populations that formed at different times at different SFRs.