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. CioniUniversity 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
Abstract
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
dex kpc-1 out to ~8 kpc from the
centre. In the SMC, [Fe/H] has a constant value of ~
dex up to ~12 kpc. The gradient of the M33
disc, until ~9 kpc, is
dex kpc-1 while
the outer disc/halo, out to ~25 kpc, has [Fe/H] 
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