EDP Sciences
Free access
Issue A&A
Volume 465, Number 2, April II 2007
Page(s) 357 - 373
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20065244

A&A 465, 357-373 (2007)
DOI: 10.1051/0004-6361:20065244

The usage of Strömgren photometry in studies of local group dwarf spheroidal galaxies

Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients,,
D. Faria1, S. Feltzing1, I. Lundström1, G. Gilmore2, G. M. Wahlgren1, A. Ardeberg1, and P. Linde1

1  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: gil@ast.cam.ac.uk

(Received 21 March 2006 / 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 $11 \times 22$ 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

© ESO 2007

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Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
Managing Editor: C. Bertout

ISSN: 0004-6361 ; e-ISSN: 1432-0746
Frequency: 12 volumes per year
Published by: EDP Sciences

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