The Lazy Giants: APOGEE Abundances Reveal Low Star Formation
  Efficiencies in the Magellanic Clouds
  We report the first APOGEE metallicities and alpha-element abundances
measured for 3600 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 on 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 with [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 -1.0<[Fe/H]<-0.5. 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". 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 star-formation efficiency of ~0.01
Gyr^-1. The MC knees are more metal poor than those of less massive Milky Way
(MW) dwarf galaxies such as Fornax, Sculptor, 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's gravitational potential only recently.
