A Tale of Three Dwarfs: No Extreme Cluster Formation in Extreme Star-forming Galaxies

Nearly all current simulations predict that outcomes of the star formation process, such as the fraction of stars that form in bound clusters (Gamma), depend on the intensity of star formation activity (sigma(SFR)) in the host galaxy. The exact shape and strength of the predicted correlations, however, vary from simulation to simulation. Observational results also remain unclear at this time, because most works have mixed estimates made from very young clusters for galaxies with higher sigma(SFR) with those from older clusters for galaxies with lower sigma(SFR). The three blue compact dwarf (BCD) galaxies ESO 185-IG13, ESO 338-IG04, and Haro 11 have played a central role on the observational side because they have some of the highest known sigma(SFR) and published values of Gamma. We present new estimates of Gamma for these BCDs in three age intervals (1-10 Myr, 10-100 Myr, 100-400 Myr), based on age-dating, which includes H alpha photometry to better discriminate between clusters younger and older than approximate to 10 Myr. We find significantly lower values for Gamma(1-10 Myr) than published previously. The likely reason for the discrepancy is that previous estimates appear to be based on age-reddening results that underestimated ages and overestimated reddening for many clusters, artificially boosting Gamma(1-10 Myr). We also find that fewer stars remain in clusters over time, with approximate to 15%-39% in 1-10 Myr clusters, approximate to 5%-7% in 10-100 Myr clusters, and approximate to 1%-2% in 100-400 Myr clusters. We find no evidence that Gamma increases with sigma(SFR). These results imply that cluster formation efficiency does not vary with star formation intensity in the host galaxy. If confirmed, our results will help guide future assumptions in galaxy-scale simulations of cluster formation and evolution.