WIDE-FIELD PLANETARY CAMERA 2 OBSERVATIONS OF LEO-A - A PREDOMINANTLYYOUNG GALAXY WITHIN THE LOCAL GROUP

The unprecedented detail of the Wide Field Planetary Camera 2 (WFPC2) color-magnitude diagrams (CMDs) of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accu rate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium -burning blue loops, and the tip of the red giant branch, we obtain a distance modulus, m - M = 24.2 +/- 0.2, or 690 +/- 60 kpc, which place s Leo A firmly within the Local Group. Our interpretation of these fea tures in the WFPC2 CMDs at this new distance based upon extremely low- metallicity (Z = 0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e., less than 2 Gyr old. A major episode of star formation 900-1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the numbe r of anomalous Cepheid variable stars seen in this galaxy. We cannot r ule out the presence of an older, underlying globular cluster age stel lar population with these data. However, using the currently available stellar evolution models, it would appear that such an older populati on is limited to no more than 10% of the total star formation to have occurred in this galaxy. Leo A provides a nearby laboratory for studyi ng young metal-poor stars and investigations of metal-poor galaxy evol ution, such as is supposed to occur for larger systems at intermediate and high redshifts.