Revealing the chemical structure of the Magellanic Clouds with APOGEE. III. Abundance gradients of the Small Magellanic Cloud

Abstract

We determine radial- and age-abundance gradients of the Small Magellanic Cloud (SMC) using spectra of 2062 red giant branch field stars observed by SDSS-IV / APOGEE-2S. With coverage out to ~ 9 kpc in the SMC, these data taken with the high resolution (R ~22 500⁠⁠) APOGEE H-band spectrograph afford the opportunity to measure extensive radial gradients for as many as 24 abundance ratios. The SMC has an overall metallicity ([Fe/H]) gradient of –0.0546 0.0043 dex kpc−1. Ages are calculated for every star to explore the evolution of the different abundance gradients. As a function of age, many gradients flatten with decreasing age but begin to steepen for ages 3.7–5.6 Gyr, particularly in the a-element [X/H] gradients. We previously detected similar evolutionary patterns ~ 1.7-3.6 Gyr earlier in the Large Magellanic Cloud (LMC) which are attributed to a recent interaction between the Magellanic Clouds. It is inferred that the feature in the SMC gradients was caused by the same interaction. The age–[X/Fe] trends, which track average [X/Fe] over time, are flat, demonstrating a slow enhancement with respect to Fe for the SMC. When comparing the SMC gradients to the LMC and MW, normalized to disc scale length (Rd⁠⁠), the [X/Fe] and [X/Mg] gradients are similar, but there is a dichotomy between the dwarfs and the Milky Way (MW) for the [X/H] gradients. The median MW [X/H] gradient around –0.119 dex/ whilst the Clouds have gradients of about –0.082 dex/Rd and –0.061 dex/Rd for the SMC and LMC, respectively.