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, the largest Milky Way (MW) 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 Cloud (MC) stars detected to date. The LMC [alpha/Fe]-[Fe/H] distribution is very flat over a large metallicity range but rises by similar to 0.1 dex at -1.0 < [Fe/H] less than or similar to -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] less than or similar to -2.2 in both MCs, implying a low star formation efficiency of similar to 0.01 Gyr(-1). The MC knees are more metal-poor than those of less massive 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. ; State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) European Union (EU) AYA2017-88254-P National Science Foundation (NSF) AST-1801940 Crafoord Foundation Swedish Research Council Ruth och Nils-Erik Stenbacks stiftelse Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE) - US National Science Foundation PHY 14-30152 Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) CONICYT FONDECYT 3180210 1170364 Becas Iberoamerica Investigador 2019, Banco Santander Chile Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) CONICYT PIA/BASAL AFB-170002 Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (PAPIIT) Universidad Nacional Autónoma de México IN109919 Alfred P. Sloan Foundation United States Department of Energy (DOE) Center for High-Performance Computing at the University of Utah