Self-consistent star formation histories of dwarf irregular galaxies

Using Hubble Space Telescope, WFPC2 photometry of the resolved stars in the nearby dwarf irregular galaxies Sextans A and the Pegasus dwarf, we have constructed color-magnitude diagrams (CMD) in V and J to a limit of 26 in V. We use the blue, core helium burning stars as a tracer of the star formation rate over the past 600 Myr in both galaxies. While Sextans A shows a relatively high star formation rate during this period, the Pegasus dwarf has been relatively quiescent. We also compare ISM abundances obtained from H II region observations using the Calar Alto 3.5-m telescope. We find that the ISM in the Pegasus dwarf shows a relatively high N/O ratio, while Sextans A shows a relatively low N/O ratio. These results are consistent with the hypothesis that oxygen production is dominated by high mass stars, while nitrogen production is dominated by intermediate to low mass stars. Thus, the star formation history as recorded in the stars is consistent with the star formation history inferred from the ISM abundances.