NOAO >   Observing Info >   Approved Programs >   2010B-0136

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Proposal Information for 2010B-0136


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

CoI: Charlie Conroy, Princeton

Title: The stellar initial mass function in elliptical galaxies

Abstract: Several studies have suggested that the initial mass function (IMF) may have been deficient in low mass stars (``bottom-light'') in the star- forming progenitors of elliptical galaxies. An IMF that varies with environment and/or redshift would have profound effects on a wide range of galaxy evolution science, including estimates of total stellar masses, star formation rates, and inferences from Galactic chemistry. So far the evidence for a non-standard IMF is circumstantial, as no study has directly measured the number of dwarf stars in present-day elliptical galaxies. Here we propose to resolve this issue by measuring the contribution of dwarf stars to the integrated light of elliptical galaxies. This is possible thanks to a weak feature in the spectra of old populations: the Wing-Ford band at 9910 \AA. This band is strong in late M main sequence stars but absent in all other types of stars. Its strength is therefore a direct measure of the contribution of \lesssim 0.4 M_\odot stars to the light. For a Salpeter (1955) IMF the Wing-Ford band is ~ 2.5 %, for a Chabrier (2003) IMF it is ~ 1 %, and for a bottom-light IMF it is \lesssim 0.5 %. We will measure the strength of the Wing-Ford band in a sample of 10 cluster early-type galaxies, with sufficient S/N to measure the band in individual galaxies with 0.3 % accuracy. The average spectrum of the 10 galaxies will enable us to reach 0.1 % accuracy and look for radial variations in the band (and therefore the IMF). Using spectra of 4 elliptical galaxies in the Coma cluster we show that we can achieve the required accuracy and S/N with the new red-sensitive detector on the Keck/LRIS spectrograph.


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