Disentangling the Stellar Populations of Fornax Dwarf Spheroidal

¹ Department of Astronomy, University of Padova, Italy
² Osservatorio Astronomico di Padova – INAF, Padova, Italy
³ Royal Observatory of Belgium, Brussels, Belgium
⁴ Joint Astronomy Centre, Hilo, Hawaii, USA
⁵ European Southern Observatory, Garching, Germany
⁶ School of Mathematics and Physics, University of Tasmania, Hobart, Australia
⁷ Research School of Astronomy and Astrophysics, Australian National University, Australia
⁸ Department of Astronomy, University of Michigan, Ann Arbor, MI, USA
⁹ Steward Observatory, University of Arizona, Tucson, USA
¹⁰ Kapteyn Astronomical Institute, Groningen, The Netherlands
¹¹ Institute of Astronomy, University of Cambridge, UK

e-mail: mary.cesetti@unipd.it

Abstract

We studied the stellar populations of Fornax dwarf spheroidal galaxy using HST/WFPC2 imaging of 7 galaxy fields. Our observations reach the oldest main-sequence turn-off, revealing distinct stars formation episodes and allowing us to address the evolution of this prototype dwarf spheroidal galaxy known to have experienced an extended history of star formation. From our HST data, spatial gradients in the stellar content of Fornax emerge with greater clarity. The outermost fields show only stars with ages between 7–12Gyr, while the intermediate region hosts a stellar population between 4–10Gyr, and stars younger of 2 Gyr are found in the innermost fields. A clearly visible gap in the subgiant branch points to bimodality in the main star formation episode. Our observations also indicate that the inner clump detected by Coleman et al. (2004) is characterized by the presence of young stars with age about 1.8 Gyr.