Star Clusters in M33. II. Global Properties

Abstract

We discuss the global physical properties of stellar clusters in M33 and compare them with LMC cluster properties. In a companion paper (Paper I) we have compiled the first unbiased, representative sample of star clusters for this galaxy from Hubble Space Telescope WFPC2 multiband images of 20 fields in M33, sampling a variety of environments from outer regions to spiral arms and central regions, as well as a range of Galactocentric distances. In this paper we fit single-mass King models to cluster surface brightness profiles and find that clusters at all ages have well-relaxed density profiles. Core radii are found to be smaller on average than those of Galactic globular clusters and LMC populous clusters at similar magnitudes. Measurements of integrated photometric properties for 60 clusters are also found in Paper I. By comparing the integrated photometric measurements with evolutionary models and theoretical M/L_V ratios, we find that clusters formed continuously in M33 from ~4×10⁶-10¹⁰ yr and have masses between ~4×10² and 3×10⁵ M_solar. There is a correlation between the ages and masses of our clusters, the oldest objects being the most massive. The global properties of the M33 clusters are compared with cluster systems of other Local Group galaxies. By studying the photometric properties (magnitudes and colors) of M33 clusters and comparing them with those of LMC clusters, a global picture of the star formation history in these two galaxies emerges.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.