Deriving physical parameters of unresolved star clusters

IV. The M 33 star cluster system^⋆

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

Received: 18 May 2015
Accepted: 29 June 2015

Abstract

Context. When trying to derive the star cluster physical parameters of the M 33 galaxy using broad-band unresolved ground-based photometry, previous studies mainly made use of simple stellar population models, shown in the recent years to be oversimplified.

Aims. In this study, we aim to derive the star cluster physical parameters (age, mass, and extinction; metallicity is assumed to be LMC-like for clusters with age below 1 Gyr and left free for older clusters) of this galaxy using models that take stochastic dispersion of cluster integrated colors into account.

Methods. We use three recently published M 33 catalogs of cluster optical broad-band photometry in standard UBVRI and in CFHT/MegaCam u^∗g′r′i′z′ photometric systems. We also use near-infrared JHK photometry that we derive from deep 2MASS images. We derive the cluster parameters using a method that takes into account the stochasticity problem, presented in previous papers of this series.

Results. The derived differential age distribution of the M 33 cluster population is composed of a two-slope profile indicating that the number of clusters decreases when age gets older. The first slope is interpreted as the evolutionary fading phase of the cluster magnitudes, and the second slope as the cluster disruption. The threshold between these two phases occurs at ~300 Myr, comparable to what is observed in the M 31 galaxy. We also model by use of artificial clusters the ability of the cluster physical parameter derivation method to correctly derive the two-slope profile for different photometric systems tested.