The Lazy Giants: APOGEE Abundances Reveal Low Star Formation Efficiencies in the Magellanic Clouds

We report the first APOGEE metallicities and alpha-abundances measured for 3800 red giant stars spanning a large radial range of both the Large (LMC) and Small Magellanic Clouds (SMC), the largest Milky Way dwarf galaxies. Our sample is an order of magnitude larger than that of previous studies and extends to much larger radial distances. These are the first results presented that make use of the newly installed southern APOGEE instrument at the du Pont telescope at Las Campanas Observatory. Our unbiased sample of the LMC spans a large range in metallicity from [Fe/H]=-0.2 to very metal poor stars of [Fe/H]=-2.5, the most metal-poor Magellanic Clouds (MCs) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range but rises by ~0.1 dex at the metal-rich end. We interpret this as a sign of the known recent increase in MC star formation activity and are able to reproduce the pattern with a chemical evolution model that includes a recent starburst. This starburst phase is required to reproduce the flat alpha-abundance distributions in both the LMC and SMC. At the metal-poor end, we capture the increase of [alpha/Fe] with decreasing [Fe/H] and constrain the alpha-knee to [Fe/H]=-2.2 in both MCs implying a low SFE of ~0.01 Gyr^-1. Counter-intuitively, the knee is more metal-poor than that of less massive Milky Way (MW) dwarf galaxies such as Fornax or Sagittarius. One possible interpretation is that the MCs formed in a lower-density environment than the MW, a hypothesis that is consistent with the paradigm that the MCs fell into the MW potential only recently.