The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

We present deep Hubble Space Telescope single-star photometry of Leo A in B,V, and I. Our new field of view is offset from the centrally located fieldobserved by Tolstoy et al. (1998) in order to expose the halo population ofthis galaxy. We report the detection of metal-poor red horizontal branch stars,which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as leastas old as metal-poor Galactic Globular Clusters which exhibit red horizontalbranches, and are considered to have a minimum age of about 9 Gyr. We discussthe distance to Leo A, and perform an extensive comparison of the data withstellar isochrones. For a distance modulus of 24.5, the data are better than50% complete down to absolute magnitudes of 2 or more. We can easily identifystars with metallicities between 0.0001 and 0.0004, and ages between about 5and 10 Gyr, in their post-main-sequence phases, but lack the detection ofmain-sequence turnoffs which would provide unambiguous proof of ancient (>10Gyr) stellar generations. Blue horizontal branch stars are above the detectionlimits, but difficult to distinguish from young stars with similar colors andmagnitudes. Synthetic color-magnitude diagrams show it is possible to populatethe blue horizontal branch in the halo of Leo A. The models also suggest ~50%of the total astrated mass in our pointing to be attributed to an ancient (>10Gyr) stellar population. We conclude that Leo A started to form stars at leastabout 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, of only 3%of Solar, in its ionized interstellar medium. The existence of old stars inthis very oxygen-deficient galaxy illustrates that a low oxygen abundance doesnot preclude a history of early star formation.