EDP Sciences
Free access
Volume 506, Number 3, November II 2009
Page(s) 1137 - 1146
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361/200912138
Published online 15 September 2009
A&A 506, 1137-1146 (2009)
DOI: 10.1051/0004-6361/200912138

The metallicity gradient as a tracer of history and structure: the Magellanic Clouds and M33 galaxies

M.-R. L. Cioni

University of Hertfordshire, Science and Technology Research Institute, Hatfield AL10 9AB, UK
    e-mail: M.Cioni@herts.ac.uk

Received 24 March 2009 / Accepted 9 August 2009

Context. The stellar metallicity and its gradient place constraints on the formation and evolution of galaxies.
Aims. This is a study of the metallicity gradient of the LMC, SMC and M33 galaxies derived from their asymptotic giant branch (AGB) stars.
Methods. The [Fe/H] abundance was derived from the ratio between C- and M-type AGB stars and its variation analysed as a function of galactocentric distance. Galaxy structure parameters were adopted from the literature.
Results. The metallicity of the LMC decreases linearly as $-0.047 \pm 0.003$ dex kpc-1 out to ~8 kpc from the centre. In the SMC, [Fe/H] has a constant value of ~ $
-1.25\pm0.01$ dex up to ~12 kpc. The gradient of the M33 disc, until ~9 kpc, is $-0.078 \pm 0.003$ dex kpc-1 while the outer disc/halo, out to ~25 kpc, has [Fe/H] $\sim -1.7$ dex.
Conclusions. The metallicity of the LMC, as traced by different populations, bears the signature of two major star forming episodes: the first one constituting a thick disc/halo population and the second one a thin disc and bar due to a close encounter with the Milky Way and SMC. The [Fe/H] of the recent episode supports an LMC origin for the Stream. The metallicity of the SMC supports star formation, ~3 Gyr ago, as triggered by LMC interaction and sustained by the bar in the outer region of the galaxy. The SMC [Fe/H] agrees with the present-day abundance in the Bridge and shows no significant gradient. The metallicity of M33 supports an “inside-out” disc formation via accretion of metal poor gas from the interstellar medium.

Key words: galaxies: abundances -- Magellanic Clouds -- Local Group -- stars: AGB and post-AGB -- galaxies: stellar content -- galaxies: individual: M33

© ESO 2009