Insights from the Outskirts: Chemical and Dynamical Properties in the outer Parts of the Fornax Dwarf Spheroidal Galaxy

Abstract: We present radial velocities and [Fe/H] abundances for 340 stars in the Fornax dwarf spheroidal from R~16,000 spectra. The targets have been obtained in the outer parts of the galaxy, a region which has been poorly studied before. Our sample shows a wide range in [Fe/H], between -0.5 and -3.0 dex, in which we detect three subgroups. Removal of stars belonging to the most metal-rich population produces a truncated metallicity distribution function that is identical to Sculptor, indicating that these systems have shared a similar early evolution, only that Fornax experienced a late, intense period of star formation (SF). The derived age-metallicity relation shows a fast increase in [Fe/H] at early ages, after which the enrichment flattens significantly for stars younger than ~8 Gyr. Additionally, the data indicate a strong population of stars around 4 Gyr, followed by a second rapid enrichment in [Fe/H]. A leaky-box chemical enrichment model generally matches the observed relation but does not predict a significant population of young stars nor the strong enrichment at late times. The young population in Fornax may therefore originate from an externally triggered SF event. Our dynamical analysis reveals an increasing velocity dispersion with decreasing [Fe/H] from sigma_sys 7.5 km/s to >14 km/s, indicating an outside-in star formation history in a dark matter dominated halo. The large velocity dispersion at low metallicities is possibly the result of a non-Gaussian velocity distribution amongst stars older than ~8 Gyr. Our sample also includes members from the Fornax GCs H2 and H5. In agreement with past studies we find [Fe/H]=-2.04+-0.04 and a mean radial velocity RV=59.36+-0.31 km/s for H2 and [Fe/H]=-2.02+-0.11 and RV=59.39+-0.44 km/s for H5. Overall, we find large complexity in the chemical and dynamical properties, with signatures that additionally vary with galactocentric distance.