Young star clusters: Metallicity tracers in external galaxies

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. These newly-formed clusters are built from recycled gas, pre-enriched to various levels within the interacting galaxies. Hence, star clusters of different ages represent a fossil record of the chemical enrichment history of their host galaxy, as well as of the host galaxy’s violent star formation history. We present a new set of evolutionary synthesis models of our galev code, specifically developed to include the gaseous emission of presently forming star clusters, and a new tool to analyse multi-color observations with our models. First results for newly-born clusters in the dwarf starburst galaxy NGC 1569 are presented. 1. Models & tests We use our evolutionary synthesis code galev to study the basic physical parameters of star clusters in interacting galaxies and violent starbursts. Star clusters can easily be approximated as simple stellar populations (SSPs), since all stars have the same age, metallicity and extinction. However, more complicated star formation / metallicity evolution histories can be studied by superimposing appropriate SSPs. The main ingredients of our code are: stellar isochrones (ensemble of stars with different masses at a given age) from the Padova group for metallicities in the range of -1.7 ≤ [Fe/H] ≤ +0.4, a stellar initial mass function (usually assumed to be Salpeter-like), the library of stellar spectra by Lejeune et al. (1997,1998), and gaseous emission (both lines and continuum). From the integrated spectra we derive magnitudes in a large number of filter systems. The gaseous line and continuum emission contributes significantly to the integrated light of stellar populations younger than 3 × 107 yr both in terms of absolute magnitudes and derived colors (Anders & Fritze v. Alvensleben 2003). The updated models are available from http://www.uni-sw.gwdg.de/∼galev/panders/ We developed a tool to compare our evolutionary synthesis models with observed cluster SEDs to determine the basic cluster parameters age, metallicity, internal extinction and mass independently. Using artificial clusters with various 1 2 P. Anders, U. Fritze v. Alvensleben & R. de Grijs