Ages and Masses of Star Clusters in M33: a Multiwavelength Study

Caitlin Moeller,Daniela Calzetti

doi:10.3847/1538-3881/ac324e

Abstract

Abstract We combine archival images for the nearby galaxy M33 (Triangulum Galaxy) from the ultraviolet (UV) to the infrared to derive ages, masses, and extinctions for the young star cluster population, and compare our physical parameters with published ones. Our goal is to test the robustness of clusters ages and masses, and possibly improve on existing ones both by expanding the wavelength range of the spectral-energy distribution (SED) fits and by using more recent population synthesis models. The rationale for this experiment is to verify the sensitivity of the clusters physical parameters to observational setups and model choices that span those commonly found in the literature. We derive the physical parameters of 137 clusters, using SEDs measured in eight UV-to-I bands, including Hα, from GALEX and ground-based images. We also add the 24 μm image from the Spitzer Space Telescope to help break some age degeneracies. We find that our derived cluster ages show significant differences with earlier determinations, while the masses remain relatively insensitive to the fitting approach adopted. We also highlight an already known difficulty in recovering old, low-extinction clusters, as SED-fitting codes tend to prefer younger, higher extinction solutions when the extinction is a free parameter. We publish updated ages, masses, and extinctions, with uncertainties for all sample star clusters, together with their photometry. Given the proximity of M33, this represents an important population to secure for the study of star formation and cluster evolution in spirals.

Highlights

Within galaxies, the structures of star formation are a continuous, scale–free hierarchy from parsecs to kiloparsecs (Lada & Lada 2003; Elmegreen 2003; Bressert et al 2010), which are expected to arise from the self–similar distribution of a turbulence–dominated ISM (Elmegreen & Efremov 1997), mediated by magnetic fields and outflow feedback (e.g., Krumholz et al 2019b)
We find that our derived cluster ages show significant differences with earlier determinations, while the masses remain relatively insensitive to the fitting approach adopted
The ages, masses, and extinctions of the 137 star clusters in M33 obtained from the photometry in Table 6 via spectral energy distribution (SED) fitting are listed in Table 5, together with the ages and masses compiled by SM2007

Summary

Introduction

The structures of star formation are a continuous, scale–free hierarchy from parsecs to kiloparsecs (Lada & Lada 2003; Elmegreen 2003; Bressert et al 2010), which are expected to arise from the self–similar distribution of a turbulence–dominated ISM (Elmegreen & Efremov 1997), mediated by magnetic fields and outflow feedback (e.g., Krumholz et al 2019b). Star clusters form in the dense regions of the hierarchy (e.g., Elmegreen 2010), and, provide a sensitive probe of the star formation process, while the rest of the hierarchy is quickly dispersed into the stellar field of the galaxy, over timescales of a few tens of Myr (Bastian 2008; Longmore et al 2014; Grasha et al 2015, 2017a). Brown & Gnedin 2021) These clusters are dispersed and randomized over short timescales ( 10 Myr) by a vast array of processes, both internal– such as gas expulsion, stellar evolution, and Moeller and Calzetti two–body relaxation,– and external,– such as tidal shear, random motions, interactions with molecular clouds, and secular evolution of the host galaxy (e.g., Gieles & Bastian 2008). The distribution of cluster masses is a sensitive tracer of formation mechanisms (e.g. Whitmore et al 1999; Larsen 2002; Gieles et al 2006b,a; Whitmore et al 2014; Johnson et al 2017; Adamo et al 2020), while the age distribution reveal the cluster disruption mechanisms (e.g., Gieles 2009; Bastian et al 2012; Silva-Villa et al 2014; Chandar et al 2016; Adamo et al 2017; Messa et al 2018)

Objectives

Results

Conclusion