The star formation histories of blue compact galaxies: Where are the GAPs?

The star formation histories of Blue Compact Dwarf (BCD) galaxies are commonly viewed as episodic: short starbursts separated by long quiescent periods. However, the near-IR color magnitude diagrams of three nearby type 'iE' BCDs obtained with the NICMOS camera aboard HST do not exhibit gaps in star formation. Stars are instead found across all mass ranges and in all of the major phases of stellar evolution (which are above our detection limits). To overcome the limitations of our data, i.e., photometric errors and incompleteness, we investigate episodic modes of star formation with the help of simulations. As a case study, we here provide simulated star formation histories of I Zw 36. We show that if we allow short stops (10-100 Myr) in star formation we can still produce acceptable color magnitude diagrams (CMD) which match the data. Once, however, the formation of stars is interruped by several 100 Myr, the morphology of the CMD changes so drastically that we conclude long gaps cannot be hidden in the data.