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

Abstract

The stellar metallicity and its gradient pose constraints to the formation and evolution of galaxies. This is a study of the metallicity gradient of the LMC, SMC and M33 galaxies derived from their asymptotic giant branch (AGB) stars. 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. The metallicity of the LMC decreases linearly as -0.047+/-0.003 dex/kpc out to ~8 kpc from the centre. In the SMC, [Fe/H] has a constant value of ~ -1.25+/-0.01 dex up to ~ 12 kpc. The gradient of the M33 disc, until ~ 9 kpc, is -0.078+/-0.003 dex/kpc while an outer disc/halo, out to ~ 25 kpc, has [Fe/H] ~ -1.7 dex. 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 MW 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.