The Star Formation & Chemical Evolution Timescales of Two Nearby Dwarf Spheroidal Galaxies

We present wide-field photometry of resolved stars in the nearby Sculptor and Fornax dwarf spheroidal galaxies, going down to the oldest Main Sequence Turn-Off. The accurately flux calibrated wide-field Colour-Magnitude Diagrams are used directly in combination with spectroscopic metallicities of individual RGB stars to constrain the ages of different stellar populations, and derive the Star Formation History with particular accuracy. The Sculptor dSph contains a predominantly ancient stellar population (>10 Gyr old), which can easily be resolved into individual stars. A galaxy dominated by an old population provides a clear view of ancient processes of galaxy formation unimpeded by overlying younger populations. The Fornax dSph is dominated by stellar populations of intermediate and young ages, which can be used to study the processes of galaxy formation in a more complex mix of stellar populations We find that the known metallicity gradients are well matched to an age gradient. This is the first time that this link with age has been directly quantified. The detailed Star Formation History shows the distribution of age with regards to the metallicity for different radii out from the centre of the galaxy. By linking the obtained SFH to observed spectroscopic abundances (alpha-elements, r- and s-process elements) of RGB stars it is possible to put ages on the chemical evolution patterns observed in this galaxy. In this way we can study the timescale of chemical evolution in these two dwarf galaxies. By comparing both dwarfs we determine whether the chemical abundance patterns seen in galaxies with recent episodes of star formation are a direct continuation of those with only old populations