Wide Field Planetary Camera 2 Observations of Leo A: A Predominantly Young Galaxy within the Local Group

The unprecedented detail of the WFPC2 colour-magnitude diagrams of theresolved stellar population of Leo A presented here allows us to determine anew distance and an accurate star formation history for this extremelymetal-poor Local Group dwarf irregular galaxy. From the position of the redclump, the helium-burning blue loops and the tip of the red giant branch, weobtain a distance modulus, m-M=24.2+/-0.2, or 690 +/- 60 kpc, which places LeoA firmly within the Local Group. Our interpretation of these features in theWFPC2 CMDs at this new distance based upon extremely low metallicity (Z=0.0004)theoretical stellar evolution models suggests that this galaxy is predominantlyyoung, i.e. <2 Gyr old. A major episode of star formation 900 - 1500 Gyr agocan explain the red clump luminosity and also fits in with our interpretationof the number of anomalous Cepheid variable stars seen in this galaxy. Wecannot rule out the presence of an older, underlying globular cluster agestellar population with these data. However, using the currently availablestellar evolution models, it would appear that such an older population islimited to no more than 10% of the total star formation to have occured in thisgalaxy. Leo A provides a nearby laboratory for studying young metal poor starsand investigations of metal-poor galaxy evolution, such as is supposed to occurfor larger systems at intermediate and high redshifts.Comment: 64 pages, 18 figures, 4 tables accepted for publication in the Astronomical Journal (Sept. 98