Systematic differences in simple stellar population model results: Application to the M31 globular-like cluster system

Simple stellar population (SSP) synthesis models are useful tools for studying the nature of unresolved star clusters in external galaxies. However, the plethora of currently available SSP models gives rise to significant and poorly documented systematic differences. Here we consider the outputs of the commonly used Bruzual & Charlot and GALEV models, as well as a recently updated SSP model suite which attempts to include the contributions of binary merger products in the form of blue straggler stars (BS-SSP). We rederive the ages, metallicities, extinction values and masses of 445 previously observed globular-like clusters in M31 based on chi-square minimisation of their spectral energy distributions with respect to these three different SSP models and adopting a Chabrier-like stellar initial mass function. A comparison between our new results and previous estimates of the same parameters shows that the Bruzual & Charlot models yield the youngest ages and lowest masses, while adoption of the BS-SSP models results in the oldest ages and highest mass estimates. Similarly, the GALEV SSP models produce the lowest metallicities, with the highest values resulting from the BS-SSP model suite. These trends are caused by intrinsic differences associated with the models, and are not significantly affected by the well-known age-metallicity degeneracy. Finally, we note that the mass function of the massive M31 star clusters is similar to that of the Milky Way's globular clusters, which implies that the two star cluster systems likely formed under similar environmental conditions.