NOAO >   Observing Info >   Approved Programs >   2011B-0274

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Proposal Information for 2011B-0274


PI: Pieter van Dokkum, Yale University, pieter.vandokkum@yale.edu
Address: Astronomy Department, 260 Whitney Ave, New Haven, CT 06511, USA

CoI: Charlie Conroy, Harvard/Smithsonian Center for Astrophysics

Title: Does the IMF vary with galaxy velocity dispersion?

Abstract: Using LRIS on Keck we recently found that very massive elliptical galaxies (with velocity dispersions \sigma>250 km/s) have enhanced Na I 8190 Aand FeH 9916 Aabsorption compared to population synthesis models and globular clusters. These absorption features are strong in dwarf stars and weak or absent in other types of stars, and the most straightforward interpretation is that the IMF in massive ellipticals had a much higher proportion of dwarf stars than the IMF in the disk of the Milky Way. A key test of this result is to measure these same absorption features in elliptical galaxies with lower velocity dispersions. Dynamical and lensing studies have shown that the mass-to-light ratios (M/L) of these ellipticals are much lower than the M/L implied by dwarf-rich IMFs, which means that their IMFs must be closer to the Milky Way IMF. Here we propose to observe a carefully selected sample of 12 well-studied elliptical galaxies with velocity dispersions in the range 100-250 km/s. The galaxies were selected to have similar ages, metal abundances, and (alpha)-enhancements as the \sigma>250 km/s galaxies we studied previously. If Na I and FeH behave like the other metal lines and do not increase with velocity dispersion, we can rule out the interpretation of van Dokkum & Conroy (2010, 2011). If, instead, Na I and FeH are decoupled from the other metals and scale with velocity dispersion, they likely reflect a systematically varying dwarf contribution with velocity dispersion.


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