Young Star Clusters in The Antennae: A Clue to their Nature from Evolutionary Synthesis

Abstract

We analyse the population of bright star clusters in the interacting galaxy pair NGC 4038/39 detected with HST WFPC1 by Whitmore & Schweizer (1995) using our spectrophotometric evolutionary synthesis models for various initial metallicities. We derive individual ages for all clusters with V-I colour and follow the time evolution of their luminosity function (LF). The age distribution clearly reveals the presence of a number of old Globular Clusters (GCs) from the parent galaxies. Their effective radii do not significantly differ from those of the young star cluster population. We confirm Meurer's (1995) conjecture that the shape of the LF changes in the course of evolution if age spread effects are accounted for. We find that over a Hubble time the observed exponential LF of the young star clusters will evolve into a typical Gaussian GCLF with parameters M$_{V_0} = -7.1$ mag and $σ(M_{V_0}) = 1.3$ mag. We discuss the influence of metallicity, the effects of an inhomogeneous internal dust distribution, as well as the possible influence of internal -- through stellar mass loss -- and external dynamical effects on the secular evolution of the LF. Referring YSC luminosities to a uniform age and combining with model M/L, we recover the intrinsic mass distribution of the YSC system. It is Gaussian in shape to good approximation thus representing a quasi-equilibrium distribution that $-$ according to Vesperini's (1997) dynamical modelling for the Milky Way GC system $-$ will not be altered in shape over a Hubble time of dynamical evolution, allthough a substantial number of clusters will be destroyed. Comparing young star cluster populations in an age sequence of interacting/merged and merger remnant galaxies will directly reveal the role of external dynamical effects.