The evolution of the chemical abundance gradients in the merging Magellanic Cloud dwarf galaxies

Abstract

Dwarf galaxies are some of the most abundant objects in the Universe, but most of them are very distant and very faint. While observing these galaxies does pose some challenges, they are important to study because it is believed that larger galaxies, such as the Milky Way (MW), form from a series of dwarf mergers in a process called hierarchical merging. As if by chance, the Magellanic Clouds (MCs) are both bright enough and close enough to resolve individual stars. These two dwarf satellites of the MW are also in the process of merging together, presenting a great opportunity to examine how the abundance gradients of galaxies are impacted by intergalactic interactions. A great tool to study the MCs is the Apache Point Galactic Evolution Experiment (APOGEE). APOGEE is an H-band near infrared survey commissioned to measure chemical abundances and accurate radial velocities of the MW and its neighborhood. In the MCs, APOGEE was able to observe 6130 red giant branch (RGB) stars in the Large Magellanic Cloud (LMC) and 2062 RGB stars in the Small Magellanic Cloud (SMC). Individual stellar ages are derived using multiband photometry and spectroscopic parameters to compare to stellar isochrones. Using the abundance measurements of 20+ elements and the derived stellar ages, abundance gradients and their evolutions are extracted from radial abundance trends. The stellar ages in the LMC reveal that recent star formation has been concentrated in the center of that galaxy. The fields that overlay a spiral arm in the north of the LMC reveal median ages of ?2 Gyr. The age-metallicity relation (AMR) remains mostly flat with the exception of an increase in overall metallicity ?2 Gyr ago. Looking at the evolution of many abundance gradients in the LMC there is a U-shaped trend with an extremum around the same time as the increase in metallicity. Additionally, the SMC also shows a U-shaped trend in its abundance gradient evolutions albeit a few billion years earlier than the LMC. These results all correspond to a conjectured close interaction between the LMC and SMC in the recent past.