The age–metallicity relationship in the Fornax spheroidal dwarf galaxy

Abstract

We produce a comprehensive field star age–metallicity relationship (AMR) from the earliest epoch until ∼1 Gyr ago for three fields in the Fornax dwarf spheroidal galaxy by using VI photometric data obtained with FORS1 at the VLT. We find that the innermost one does not contain dominant very old stars (age > 12 Gyr), whereas the relatively outer field does not account for representative star field populations younger than ∼3 Gyr. When focusing on the most prominent stellar populations, we find that the derived AMRs are engraved by the evidence of an outside-in star formation process. The studied fields show bimodal metallicity distributions peaked at [Fe/H] = (−0.95 ± 0.15) dex and (−1.15 or −1.25 ± 0.05) dex, respectively, but only during the first half of the entire galaxy lifetime. Furthermore, the more metal-rich population appears to be more numerous in the outer fields, while in the innermost Fornax field the contribution of both metallicity populations seems to be similar. We also find that the metallicity spread ∼6 Gyr ago is remarkable large, while the intrinsic metallicity dispersion at ∼1–2 Gyr results smaller than that for the relatively older generations of stars. We interpret these outcomes as a result of a possible merger of two galaxies that would have triggered a star formation bursting process that peaked between ∼6 and 9 Gyr ago, depending on the position of the field in the galaxy.