The star formation history of LGS 3

We have determined the distance and star formation history of the Local Gro up dwarf galaxy LGS 3 from deep Hubble Space Telescope WFPC2 observations. LGS 3 is intriguing because ground-based observations showed that, while it s stellar population is dominated by old, metal-poor stars, there is a hand ful of young, blue stars. Also, the presence of H I gas makes this a possib le "transition object" between dwarf spheroidal and dwarf irregular galaxie s. The HST data are deep enough to detect the horizontal branch and young m ain sequence for the first time. A new distance of D = 620 +/- 20 kpc has b een measured from the positions of the tip of the red giant branch, the red clump, and the horizontal branch. The mean metallicity of the stars older than 8 Gyr is [Fe/H] = -1.5 +/- 0.3. The most recent generation of stars ha s [Fe/H] approximate to -1. For the first few Gyr the global star formation rate was several times higher than the historical average and has been fai rly constant since then. However, we do see significant changes in stellar populations and star formation history with radial position in the galaxy. Most of the young stars are found in the central 63 pc (21 "), where the st ar formation rate has been relatively constant, while the outer parts have had a declining star formation rate.