Probing cluster formation under extreme conditions: massive star
  clusters in blue compact galaxies
  The numerous and massive young star clusters in blue compact galaxies (BCGs)
are used to investigate the properties of their hosts. We test whether BCGs
follow claimed relations between cluster populations and their hosts, such as
the the fraction of the total luminosity contributed by the clusters as
function of the mean star formation rate density; the $V$ band luminosity of
the brightest youngest cluster as related to the mean host star formation rate;
and the cluster formation efficiency (i.e., the fraction of star formation
happening in star clusters) versus the density of the SFR. We find that BCGs
follow the trends, supporting a scenario where cluster formation and
environmental properties of the host are correlated. They occupy, in all the
diagrams, the regions of higher SFRs, as expected by the extreme nature of the
starbursts operating in these systems. We find that the star clusters
contribute almost to the 20 % of the UV luminosity of the hosts. We suggest
that the BCG starburst environment has most likely favoured the compression and
collapse of the giant molecular clouds, enhancing the local star formation
efficiency, so that massive clusters have been formed. The estimated cluster
formation efficiency supports this scenario. BCGs have a cluster formation
efficiency comparable to luminous IR galaxies and spiral starburst nuclei (the
averaged value is about 35 %) which is much higher than the 8 - 10 % reported
for quiescent spirals and dwarf star-forming galaxies.
