Newly identified star clusters in M33 - III. Structural parameters

We present the morphological properties of 161 star clusters in M33 using the Advanced Camera for Surveys Wide Field Channel on-board the Hubble Space Telescope using observations with the F606W and F814W filters. We obtain, for the first time, ellipticities, position angles and surface brightness profiles for a significant number of clusters. On average, M33 clusters are more flattened than those of the Milky Way (MW) and M31, and more similar to clusters in the Small Magellanic Cloud (SMC). The ellipticities do not show any correlation with age or mass, suggesting that rotation is not the main cause of elongation in the M33 clusters. The position angles of the clusters show a bimodality with a strong peak perpendicular to the position angle of the galaxy major axis. These results support the notion that tidal forces are the reason for the cluster flattening. We fit King and Elson-Freeman-Fall (EFF) models to the surface brightness profiles and derive structural parameters including core radii, concentration, half-light radii and central surface brightness for both filters. The surface brightness profiles of a significant number of clusters show irregularities such as bumps and dips. Young clusters (log age < 8) are notably better fitted by models with no radial truncation (EFF models), while older clusters show no significant differences between King and EFF fits. M33 star clusters seem to have smaller sizes, smaller concentrations and smaller central surface brightness as compared to clusters in the MW, M31, Large Magellanic Cloud and SMC. Analysis of the structural parameters presents an age-radius relation also detected in other star cluster systems. The overall analysis shows differences in the structural evolution between the M33 cluster system and cluster systems in nearby galaxies. These differences could have been caused by the strong differences in these various environments. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.