Blue compact dwarf galaxies - Home to the latest of the first stars?

We review single-star photometry with the Hubble Space Telescope (HST) of five well resolved Blue Compact Dwarf (BCD) galaxies. Three BCDs exhibit well populated red giant branches. These galaxies are therefore at least 1-2 Gyr old, and possibly older than 10 Gyr. For two BCDs, the available HST data do not go deep enough to reveal the red giant branch. We argue that over a wide range of possible galaxy distances, the color-magnitude diagrams of these BCDs are nevertheless consistent with showing the evolved descendants of intermediate-mass stars on the asymptotic giant branch. Such stars are nucleosynthetic sites of nitrogen and carbon production. Their identification in BCDs contradicts recent conclusions that BCDs must be young galaxies in which primary production of these elements takes place exclusively in massive stars, and implies that BCDs are unlikely to be home to newly formed metal-free stars. Color-magnitude diagrams of BCDs thus point to difficulties in decoding star-formation histories of starburst galaxies from the metallicities of the ionized gas.