Deep Wide-Field Imaging down to the oldest Main Sequence Turnoffs in the
  Sculptor dwarf spheroidal galaxy
  We present wide-field photometry of resolved stars in the nearby Sculptor
dwarf spheroidal galaxy using CTIO/MOSAIC, going down to the oldest Main
Sequence Turn-Off. The accurately flux calibrated wide field Colour-Magnitude
Diagrams can be used to constrain the ages of different stellar populations,
and also their spatial distribution. The Sculptor dSph contains a predominantly
ancient stellar population (>10 Gyr old) which can be easily 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. By using spectroscopic metallicities of RGB stars in combination
with our deep Main Sequence Turn-Off photometry we can constrain the ages of
different stellar populations with particular accuracy. We find that the known
metallicity gradient in Sculptor is well matched to an age gradient. This is
the first time that this link with age has been directly quantified. This
gradient has been previously observed as a variation in Horizontal Branch
properties and is now confirmed to exist for Main Sequence Turn-Offs as well.
It is likely the Sculptor dSph first formed an extended metal-poor population
at the oldest times, and subsequent more metal-rich, younger stars were formed
more towards the centre until the gas was depleted or lost roughly 7 Gyr ago.
The fact that these clear radial gradients have been preserved up to the
present day is consistent with the apparent lack of signs of recent tidal
interactions.
