Using deep V , B - V wide-field photometry , we have conducted a new study of stellar over-densities in the Fornax dwarf spheroidal galaxy by determining detailed Star Formation Histories from Colour-Magnitude Diagram analysis .
We have concentrated on the relatively young stellar component ( < 4 Gyr old ) , and compared this to the underlying Fornax field population . We have studied in more detail the previously known inner shell-like structure and shown that it has a well-defined age-metallicity relation with a peak at \approx 1.5 Gyr , [ Fe/H ] = - 0.6 dex .
Comparison to the Fornax centre shows that the over-dense feature is consistent with the age-metallicity relation of young field stars , and likely formed from Fornax gas .
This is consistent with a scenario in which the over-density was formed by the re-accretion of previously expelled gas . We have also discovered a new stellar over-density , located 0.3 degrees ( 0.7 kpc ) from the centre , which is only 100 Myr old , with solar metallicity .
This feature constitutes some of the youngest , most metal-rich stars stars observed in Fornax to date .
It is unclear how the young over-density was formed , although the age and metallicity of stars suggest this feature may represent the last star formation activity of the Fornax dSph .