Is I ZW 18 Really a Young Galaxy?

To understand the role of dwarfs in the context of galaxy formation and evolution, we are studying the star-formation history of some representative irregular and blue compact dwarf galaxies by comparing the observed colour–magnitude diagrams with synthetic ones based on homogeneous sets of stellar evolutionary tracks. Here we present the results obtained for the blue compact I Zw 18, the most metal-poor galaxy known. Our simulations suggest star-formation activity started around 1–0.3 Gyr ago, with evidence of an intense burst around 15–20 Myr ago. I Zw 18 has turned out not to be a young object as previously suggested. This revised version was published online in July 2006 with corrections to the Cover Date.