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