Deriving physical parameters of unresolved star clusters

II. The degeneracies of age, mass, extinction, and metallicity

¹ Vilnius University Observatory, Čiurlionio 29, 03100 Vilnius, Lithuania
e-mail: philippe.demeulenaer@ff.stud.vu.lt
² Center for Physical Sciences and Technology, Savanorių 231, 02300 Vilnius, Lithuania

Received: 14 April 2014
Accepted: 26 June 2014

Abstract

Context. This paper is the second of a series that investigates the stochasticity and degeneracy problems that hinder the derivation of the age, mass, extinction, and metallicity of unresolved star clusters in external galaxies when broad-band photometry is used.

Aims. While Paper I concentrated on deriving age, mass, and extinction of star clusters for one fixed metallicity, we here derive these parameters in case when metallicity is let free to vary. The results were obtained using several different filter systems (UBVRI, UBVRIJHK, GALEX+UBVRI), which allowed us to optimally reduce the different degeneracies between the cluster physical parameters.

Methods. The age, mass, and extinction of a sample of artificial star clusters were derived by comparing their broad-band integrated magnitudes with the magnitudes of a large grid of cluster models with various metallicities.

Results. A large collection of artificial clusters was studied to model the different degeneracies in the age, mass, extinction, and metallicity parameter space when stochasticity is taken into account in the cluster models. We show that, without prior knowledge on the metallicity, the optical bands (UBVRI) fail to allow a correct derivation of the age, mass, and extinction because of the strong degeneracies between models of different metallicities. Adding near-infrared information (UBVRI+JHK) slightly helps in improving the parameter derivation, except for the metallicity. Adding ultraviolet data (GALEX+UBVRI) helps significantly in deriving these parameters and allows constraining the metallicity when the photometric errors have a Gaussian distribution with standard deviations 0.05 mag for UBVRI and 0.15 mag for the GALEX bands.