A Tale of Three Dwarfs: Cluster-based Star Formation Histories of Blue Compact Dwarf Galaxies

Abstract

We present a new study of the cluster populations in the blue compact dwarf galaxies (BCD) ESO185-IG13, ESO338-IG04, and Haro11, based on new and archival high-resolution images taken by the Hubble Space Telescope, and the first to probe the populations older than ≈100 Myr. BCDs are believed to experience intense bursts of star formation (including at the present day) after long periods of quiescence, but little is known about the timing, frequency, duration, and strength of these bursts or about their star formation histories in general. We find that the cluster population in each of the three galaxies studied here has its own unique distribution of colors and hence a unique cluster and star formation history. From an assumed correlation between the normalization of the cluster mass function and the star formation rate of the host galaxy, we construct cluster-based star formation histories over the past ≈few × Gyr and find that only Haro11 is currently experiencing a burst (≈factor of 10 increase in the rate of star formation for the last ≈20 Myr), whereas ESO185 experienced enhanced star formation (by a factor ≈4) between 10 and 40 Myr ago, and ESO338 has had a fairly constant SFH over the past few Gyr. These findings indicate that not all BCDs are experiencing a burst of star formation at the present day, and that some have been forming stars and clusters at a fairly steady rate (within a factor of ≈2–3) over the past few Gyr. This scenario is similar to the histories of dwarf irregular and dwarf starburst galaxies, which have star formation rates that are 10–1000 times lower than those in BCDs.