Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients
Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden e-mail: [daniel;sofia;ingemar;glenn;arne;peter]@astro.lu.se
2 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK e-mail: email@example.com
Accepted: 2 August 2006
Aims.In this paper we demonstrate how Strömgren uvby photometry can be efficiently used to: 1. identify red giant branch stars that are members in a dwarf spheroidal galaxy; 2. derive age-independent metallicities for the same stars and quantify the associated errors.
Methods.Strömgren uvby photometry in a arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were identified using the surface gravity sensitive c1 index which discriminates between red giant and dwarf stars. Thus enabling us to distinguish the (red giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy. The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions. The metallicity sensitive m1 index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy. The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good.
Results.We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy (the largest sample to date). The metallicity distribution function for the Draco dSph galaxy shows a mean [Fe/H] = –1.74 dex with a spread of 0.24 dex. The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old, and coeval population. There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars.
Key words: galaxies: dwarf / galaxies: individual: Draco dSph / galaxies: individual: UGC 10822 / galaxies: stellar content / galaxies: photometry / Local Group
Based on observations made with the Isaac Newton Telescope, operated on the Island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
Guest User, Canadian Astronomy Data Centre, which is operated by the Herzberg Institute of Astrophysics, National Research Council of Canada.
© ESO, 2007