Author(s): D. Nidever, Physics, Montana State University, Bozeman, Montana, UNITED STATES|D. Nidever, V. Smith, NOAO, Tucson, Arizona, UNITED STATES|S. Hasselquist, University of Utah, Salt Lake, Utah, UNITED STATES|C.R. Hayes, S. Majewski, B. Anguiano, University of Virginia, Charlottesville, Virginia, UNITED STATES|G. Stringfellow, CU Boulder, Boulder, Colorado, UNITED STATES|
Institution(s): 1. Physics, Montana State University, Bozeman, MT, United States. 2. NOAO, Tucson, AZ, United States. 3. University of Utah, Salt Lake, UT, United States. 4. University of Virginia, Charlottesville, VA, United States. 5. CU Boulder, Boulder, CO, United States.
Contributing team(s): SDSS-IV/APOGEE
How galaxies form and evolve remains one of the cornerstone questions in our understanding of the universe on grand scales. While much progress has been made in understanding the formation and chemical evolution of larger galaxies by studying the Milky Way and other L* galaxies in the Local Volume, our knowledge of the evolution of dwarf galaxies, especially their chemical enrichment histories, is far more limited because these systems are intrinsically faint, and offer access to significantly fewer resolved stars. The SDSS-IV/APOGEE survey is dramatically improving this situation through its large spectroscopic census of 5,000 giant stars in the nearby Magellanic Clouds (MCs) spanning a large range of radii and position angles. To date, high-S/N spectra of over 3,000 giant stars have been obtained. Using these data, we identify a significant fraction of metal-poor stars in the Large MC (LMC) and make the first clear detection of the [alpha/Fe]-[Fe/H] "knee” — a drop in the abundance of alpha-elements relative to iron due to the increased iron production in SNIa — at [Fe/H] = -2.2. The LMC knee is more metal-poor (indicating a low star formation efficiency) than those of less massive MW dwarf galaxies (e.g., Fornax, Sagittarius), a counter-intuitive result that suggests the LMC likely formed in a lower-density environment — but one that is, however, consistent with the recently developed paradigm that the LMC only recently fell into the MW potential. We also make the first large-scale elemental abundance maps of the MCs, which enable us to compare directly to APOGEE-derived abundance gradients of the Milky Way. Interestingly, the LMC spatial abundance gradients are significantly smaller than those in the MW suggesting that the LMC's star forming gas was better mixed over the last few gigayears.