The DART imaging and CaT survey of the Fornax dwarf spheroidal galaxy

¹ Kapteyn Institute, University of Groningen, Postbus 800, 9700AV Groningen, The Netherlands e-mail: gbattag1@astro.rug.nl
² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Observatoire de Genève, Laboratoire d'Astrophysique de l'École Polytechnique Fédérale de Lausanne (EPFL), 1290 Sauverny, Switzerland
⁴ Observatoire de Paris-Meudon, 92195 Meudon Cedex, France
⁵ Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 1A1, Canada
⁶ University of Texas, McDonald Observatory, USA
⁷ National Astronomical Observatory, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸ European Southern Observatory, Karl-Schwarzschild str 2, 85748 Garching bei München, Germany
⁹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
¹⁰ Kavli Institute for Particle Astrophysics and Cosmology, Department of Physics and Stanford Linear Accelerator Center, Stanford University, 2575 Sand Hill Road, Menlo Park, CA 94044, USA
¹¹ Astronomical Institute, Osaka Kyoiku University, Kashiwara-shi, Osaka 581-8582, Japan

Received: 29 May 2006
Accepted: 26 July 2006

Abstract

Aims.As part of the DART project we have used the ESO/2.2m Wide Field Imager in conjunction with the VLT/FLAMES GIRAFFE spectrograph to study the detailed properties of the resolved stellar population of the Fornax dwarf spheroidal galaxy out to and beyond its tidal radius. Fornax dwarf spheroidal galaxy has had a complicated evolution and contains significant numbers of young, intermediate age and old stars. We investigate the relation between these different components by studying their photometric, kinematic and abundance distributions.

Methods. We re-derived the structural parameters of the Fornax dwarf spheroidal using our wide field imaging covering the galaxy out to its tidal radius, and analysed the spatial distribution of the Fornax stars of different ages as selected from colour–magnitude diagram analysis. We have obtained accurate velocities and metallicities from spectra in the Ca II triplet wavelength region for 562 Red Giant Branch stars which have velocities consistent with membership of the Fornax dwarf spheroidal.

Results.We have found evidence for the presence of at least three distinct stellar components: a young population (few 100 Myr old) concentrated in the centre of the galaxy, visible as a Main Sequence in the colour–magnitude diagram; an intermediate age population (2-8 Gyr old); and an ancient population (>10 Gyr), which are distinguishable from each other kinematically, from the metallicity distribution and in the spatial distribution of stars found in the colour–magnitude diagram.

Conclusions. From our spectroscopic analysis we find that the “metal rich” stars () show a less extended and more concentrated spatial distribution, and display colder kinematics than the “metal poor” stars (). There is tentative evidence that the ancient stellar population in the centre of Fornax does not exhibit equilibrium kinematics. This could be a sign of a relatively recent accretion of external material, such as the merger of another galaxy or other means of gas accretion at some point in the fairly recent past, consistent with other recent evidence of substructure (Coleman et al. 2004, AJ, 127, 832; 2005, AJ, 129, 1443).