Hubble Space Telescope photometry of the Fornax dwarf spheroidal galaxy: Cluster 4 and its field

Using observations from the Hubble Space Telescope archive, color-magnitude diagrams (CMDs) have been constructed for globular cluster 4 in the Fornax dwarf spheroidal galaxy and its surrounding field. These diagrams extend b elow the main-sequence turnoffs and have yielded measurements of the ages o f the populations. The most prominent features of the CMD of the Fornax hel d population are a heavily populated red clump of horizontal-branch (HB) st ars, a broad red giant branch (RGB), and a main sequence that spans a large range in luminosity. In this CMD, there are very few stars at the position s of the HBs of the five globular clusters in Fornax, which suggests that o nly a very small fraction of the field population resembles the clusters in age and chemical composition. The large span in luminosity of the main seq uence suggests that star formation began in the field similar or equal to 1 2 Gyr ago and continued to similar or equal to 0.5 Gyr ago. There are separ ate subgiant branches in the CMD, which indicates that the star formation w as not continuous but occurred in bursts. The CMD of cluster 4 has a steep RGB, from which we estimate [Fe/H] similar or equal to -2.0. This is consid erably lower than estimates from the integrated light of the cluster, and t he origins of this discrepancy are discussed. Cluster 4 has a very red HE a nd is, therefore, a prime example of the second-parameter effect. Compariso ns of cluster 4 with the other Fornax clusters and with M68, a very metal-p oor globular duster of the Galactic halo, reveal that cluster 4 is similar or equal to 3 Gyr younger than these other clusters, which have much bluer HBs. This age difference is consistent with the prediction that age is the second parameter to within the uncertainties. The CMD of cluster 4 is virtu ally identical to that of the unusual globular cluster of the Galactic halo Ruprecht 106, which suggests that they have very similar ages and chemical compositions. We discuss the possibility that cluster 4 also resembles R10 6 in having a higher [Fe/H] than is indicated by its steep RGB and also a l ower [alpha/Fe] ratio than is usual for a globular cluster, as indicated by some recent observations of R106. The CMDs of the five Fornax clusters ind icate that cluster age is a major-but probably not the sole-second paramete r. Buonanno et al. recently concluded that cluster density probably influen ced the HE morphologies of clusters 1, 2, 3, and 5. Despite a very large di fference in central density, the HBs of cluster 4 and R106 are very similar . This suggests that density may act as a second parameter in clusters that have HBs that are on the verge of moving toward the blue or are already bl ue for another reason, such as very old age.