2009ApJ...691L..59W

Based on deep imaging from the Advanced Camera for Surveys aboard the Hubble Space Telescope (HST), we present new evidence that stellar feedback created a ∼1 kpc supergiant H I shell (SGS) and triggered star formation (SF) around its rim in the M81 Group dwarf irregular galaxy IC 2574. Using photometry of the resolved stars from the HST images, we measure the star formation history of a region including the SGS, focusing on the past 500 Myr, and employ the unique properties of blue helium-burning stars to create a movie of SF in the SGS. We find two significant episodes of SF inside the SGS from 200-300 Myr and ∼25 Myr ago. Comparing the timing of the SF events to the dynamic age of the SGS and the energetics from the H I and SF, we find compelling evidence that stellar feedback is responsible for creating the SGS and triggering secondary SF around its rim.