Insights from the outskirts: Chemical and dynamical properties in the outer parts of the Fornax dwarf spheroidal galaxy^⋆,⋆⋆

¹ Zentrum für Astronomie der Universität Heidelberg, Landessternwarte, Königstuhl 12, 69117 Heidelberg, Germany
e-mail: ben.hendricks@lsw.uni-heidelberg.de
² McWilliams Center for Cosmology, Carnegie Mellon University, 5000 Forbes Ave., Pittsburgh, PA 15213, USA
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-15, Cambridge, MA 02138, USA
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ Institute of Astronomy, Cambridge University, Madingley Rd, Cambridge CB3 OHA, UK

Received: 21 July 2014
Accepted: 22 August 2014

Abstract

We present radial velocities and [Fe/H] abundances for 340 stars in the Fornax dwarf spheroidal from R ~ 16 000 spectra. The targets were obtained in the outer parts of the galaxy, a region that has been poorly studied. 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 shared a similar early evolution, except 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 predicts neither a significant population of young stars nor strong enrichment at late times. The young population in Fornax may therefore stem from an externally triggered SF event. Our dynamical analysis reveals an increasing velocity dispersion with decreasing [Fe/H] from σ_sys ≈ 7.5 km s^-1 to ≥ 14 km s^-1. The large velocity dispersion at low metallicities is possibly the result of a non-Gaussian velocity distribution among stars older than ~ 8 Gyr. Our sample also includes members from the Fornax globular clusters 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^-1 for H2 and [Fe/H] = −2.02 ± 0.11 and RV = 59.39 ± 0.44 km s^-1 for H5. Finally, we test different calibrations of the calcium triplet over more than 2 dex in [Fe/H] and find best agreement with the calibration equations provided by Carrera et al. (2013, MNRAS, 434, 1681). Overall, we find high complexity in the chemical and dynamical properties, with signatures that additionally vary with galactocentric distance. Detailed knowledge about the properties of stars at all radii is therefore needed to draw a conclusive picture about the star formation and chemical evolution in Fornax.