A Tale of Three Dwarfs: No Extreme Cluster Formation in Extreme Star-forming Galaxies
Rupali Chandar, Miranda Caputo, Angus Mok, Sean T. Linden, Bradley C. Whitmore, Aimee Toscano, Jaidyn Conyer, D. Cook, Janice Lee, Leonardo Úbeda, R. L. White
Abstract
Abstract Nearly all current simulations predict that outcomes of the star formation process, such as the fraction of stars that form in bound clusters (Γ), depend on the intensity of star formation activity (Σ 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 Σ SFR with those from older clusters for galaxies with lower Σ 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 Σ SFR and published values of Γ. We present new estimates of Γ for these BCDs in three age intervals (1–10 Myr, 10–100 Myr, 100–400 Myr), based on age-dating, which includes H α photometry to better discriminate between clusters younger and older than ≈10 Myr. We find significantly lower values for Γ(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 Γ(1–10 Myr). We also find that fewer stars remain in clusters over time, with ≈15%–39% in 1–10 Myr clusters, ≈5%–7% in 10–100 Myr clusters, and ≈1%–2% in 100–400 Myr clusters. We find no evidence that Γ increases with Σ 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.