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

TL;DR

This study investigates the metallicity gradients in the Magellanic Clouds and M33 galaxies using AGB stars, revealing insights into their formation history, structure, and interactions with the Milky Way.

Contribution

It provides detailed measurements of metallicity gradients in LMC, SMC, and M33, linking these to their formation and interaction histories, using a novel approach with AGB star ratios.

Findings

LMC metallicity decreases linearly with radius.

SMC metallicity remains constant up to 12 kpc.

M33 shows an inside-out formation pattern.

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.