Comparisons of Different Fitting Methods for the Physical Parameters of a Star Cluster Sample of M33 with Spectroscopy and Photometry

Star clusters are good tracers for formation and evolution of galaxies. We compared different fitting methods by using spectra (or by combining photometry) to determine the physical parameters. We choose a sample of 17 star clusters in M33, which previously lacked spectroscopic observations. The low-resolution spectra were taken with the Xinglong 2.16 m reflector of NAOC. The photometry used in the fitting includes u_SC and v_SAGE bands from the SAGE survey, as well as the published UBVRI and ugriz photometry. We first derived ages and metallicities with the ULYSS (Vazdekis et al. and PEGASE-HR) simple stellar population model and the Bruzual & Charlot stellar population synthesis models for the full-spectrum fitting. The fitting results of both the BC03 and ULYSS models seem consistent with those of previous works as well. Then, we add the SAGE u_SC and v_SAGE photometry in the spectroscopic fitting with the BC03 models. It seems that the results become much better, especially for the Padova 2000+Chabrier initial mass function set. Finally, we add more photometry data, UBVRI and ugriz, in the fitting, and we found that the results do not improve significantly. Therefore, we conclude that the photometry is useful for improving the fitting results, especially for the blue bands (λ < 4000 Å), e.g., u_SC and v_SAGE band. At last, we discuss the "UV excess" for the star clusters, and we find that five star clusters have UV excess, based on the GALEX far-UV and near-UV photometry.