Deriving physical parameters of unresolved star clusters IV. The M33 star cluster system

TL;DR

This study derives physical parameters of unresolved star clusters in M33 using stochastic models, revealing a two-phase age distribution with fading and disruption phases, and compares different photometric systems for accuracy.

Contribution

It introduces a method that accounts for stochastic color dispersion in deriving cluster parameters, improving over previous oversimplified models.

Findings

The age distribution shows a two-slope profile with a transition at ~300 Myr.

The method accurately recovers the two-phase distribution using artificial clusters.

The transition point is consistent with observations in M31.

Abstract

Context. When trying to derive the star cluster physical parameters of the M33 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 M33 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 M33 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 $\sim$300\,Myrs, comparable to what is observed in the M31 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.