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 Ðrst unbiased, representative sample of star clusters for this galaxy from Hubble Space Telescope WFPC2 multiband images of 20 Ðelds 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 Ðt single-mass King models to cluster surface brightness proÐles and Ðnd that clusters at all ages have well-relaxed density proÐles. 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 theo- retical M ratios, we Ðnd that clusters formed continuously in M33 from D4]106È1010 yr and have /L V masses between D4]102 and 3]105 M 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 com- pared 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.