The metallicity gradient as a tracer of history and structure: the Magellanic Clouds and M33 galaxies
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 ± 0.003 dex kpc-1 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-1 while the outer disc/halo, out to ~25 kpc, has [Fe/H] ̃ -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.
Astronomy and Astrophysics
- Pub Date:
- November 2009
- galaxies: abundances;
- Magellanic Clouds;
- Local Group;
- stars: AGB and post-AGB;
- galaxies: stellar content;
- galaxies: individual: M33;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- 12 pages, 6 figures, accepted version