Star Cluster Demographics. I. A General Framework and Application to the Antennae Galaxies

Abstract

We present a framework for understanding the demographics of star cluster systems, and develop a toy model which incorporates a universal initial power law mass function, selected formation histories, selected disruption laws, and a convolution with common artifacts and selection effects found in observational data. The model confirms that the observed correlation between the brightest young cluster in a galaxy and the total number of young clusters can be understood as a statistical size-of-sample effect, rather than a difference in the physical process responsible for the formation of the clusters. A comparison is made between different cluster disruption laws and it is shown that the break in the dN/dτdiagram used to determine the parameters in the Boutloukos & Lamers model may be produced by incompleteness near the breakpoint. A model of the Antennae galaxies is developed and compared with the observational data. An important component of our model is the use of a "two-stage" disruption process, with a very high "infant mortality" rate for the clusters with ages less than \approx10^8 yrs (i.e., roughly 80%-90% are lost each factor of ten in time, τ, independent of mass), and two-body relaxation, which becomes the dominant disruption mechanism at older ages, preferentially removing the lower mass clusters. Hence, in our model, stars from the dissolved clusters form the field population. We note that a 90% infant mortality rate for each factor of ten in τ(i.e., dN/dτ\propto τ^{-1}) is consistent with all measured young cluster populations, including those in the Antennae, Small Magellanic Cloud, and the Milky Way.