The ages of the globular clusters in the Fornax dwarf galaxy

From observations with the Hubble Space Telescope, color-magnitude diagrams (CMDs) have been constructed for the four most metal-poor globular clusters in the Fornax dSph galaxy. These diagrams confirm that cluster 1 has a redder horizontal-branch (HB) morphology than the other three clusters despite its similarity in metallicity (the second parameter effect). The near coincidence of the fiducial lines of the CMDs indicates that the ages of the Fornax clusters are identical to within 1 Gyr. Since a larger range is predicted if age is the second parameter, we conclude that either the sensitivity of the HE to age has been underestimated or some parameter besides age is acting as a second parameter. The Fornax clusters exhibit a correlation between HE morphology and central density that is similar to the one among galactic globular clusters, which suggests that some mechanism related to density is influencing HE morphology. No significant differences in age have been found between the Fornax clusters and the metal-poor galactic globular clusters M68 and M92, which suggests that cluster formation began nearly simultaneously over a large volume of space.