Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst★

Abstract

We present a HST multiband analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. We have reconstructed the spectral energy distributions of the star clusters and estimated age, mass, and extinction for a representative sample. We observe a very young cluster population with 70% of the systems formed less than 10 Myr ago. The peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and IR lines ratios [OIII]/H_beta and [NeIII]/[NeII]. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons). We map the extinction across the galaxy using low-resolution spectra and the H_alpha/H_beta ratio, as obtained from ground-based narrow band imaging. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters, but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths remains unknown. We compare the cluster and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties we perform a study of the host environmental properties. We find that the cluster formation efficiency is significantly higher, suggesting a key role of the environment for the formation of these massive objects.[Abridged]