The age-metallicity relationship in the Fornax spheroidal dwarf galaxy

TL;DR

This study reconstructs the detailed age-metallicity relationship in the Fornax dwarf galaxy, revealing outside-in star formation, bimodal metallicity distributions, and evidence of past galaxy mergers influencing its stellar populations.

Contribution

It provides the first comprehensive AMR for multiple fields in Fornax, highlighting spatial variations and the impact of merger events on its star formation history.

Findings

Outer fields lack very old stars (>12 Gyr).

Bimodal metallicity distributions are present early in the galaxy.

Metallicity spread was largest around 6 Gyr ago.

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 dSph galaxy by using VI photometric data obtained with FORS1 at the VLT. We find that the innermost one does not contains 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 a 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.