Red giants in the Small Magellanic Cloud - II. Metallicity gradient and age-metallicity relation

We present results from the largest Ca ii triplet line metallicity study of Small Magellanic Cloud (SMC) field red giant stars to date, involving 3037 objects spread across approximately 37.5 deg², centred on this galaxy. We find a median metallicity of [Fe/H] = −0.99 ± 0.01, with clear evidence for an abundance gradient of −0.075 ± 0.011 dex deg⁻¹ over the inner 5°. We interpret the abundance gradient to be the result of an increasing fraction of young stars with decreasing galactocentric radius, coupled with a uniform global age–metallicity relation. We also demonstrate that the age–metallicity relation for an intermediate-age population located 10 kpc in front of the north-east of the cloud is indistinguishable from that of the main body of the galaxy, supporting a prior conjecture that this is a stellar analogue of the Magellanic Bridge. The metal-poor and metal-rich quartiles of our red giant branch star sample (with complementary optical photometry from the Magellanic Clouds Photometric Survey) are predominantly older and younger than approximately 6 Gyr, respectively. Consequently, we draw a link between a kinematical signature, tentatively associated by us with a disc-like structure, and the upsurges in stellar genesis imprinted on the star formation history of the central regions of the SMC. We conclude that the increase in the star formation rate around 5–6 Gyr ago was most likely triggered by an interaction between the SMC and Large Magellanic Cloud.