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

摘要

Context. The stellar metallicity and its gradientplace constraints on the formation and evolution of galaxies. Aims. This is a study of the metallicity gradient ofthe LMC, SMC and M33 galaxies derived from their asymptotic giantbranch (AGB) stars. Methods. The [Fe/H] abundance was derived from theratio between C- and M-type AGB stars and its variation analysed as afunction of galactocentric distance. Galaxy structure parameters wereadopted from the literature. Results. The metallicity of the LMC decreaseslinearly as dexkpc-1out to 8kpcfrom the centre. In the SMC, [Fe/H] has a constant value of dex up to 12kpc.The gradient of the M33 disc, until 9kpc, is dexkpc-1while the outer disc/halo, out to 25kpc, has[Fe/H]dex.Conclusions. The metallicity of the LMC, as tracedby different populations, bears the signature of two major star formingepisodes: the first one constituting a thick disc/halo population andthe second one a thin disc and bar due to a close encounter with theMilky Way and SMC. The [Fe/H] of the recent episode supports an LMCorigin for the Stream. The metallicity of the SMC supports starformation, 3Gyrago, as triggered by LMC interaction and sustained by the bar in theouter region of the galaxy. The SMC[Fe/H] agrees with thepresent-day abundance in the Bridge and shows no significant gradient.The metallicity of M33 supports an ``inside-out'' disc formation viaaccretion 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