(Proposal ID) S14A-001

(PI) Smith, Russell

(Proposal Title)
 The IMF and sodium abundance trends in passive Coma Cluster galaxies

(Abstract)
Recent spectroscopic studies have found evidence that the stellar initial mass 
function (IMF) in red-sequence galaxies may be quite different from that in the
 Milky Way. The most massive ellipticals show unexpectedly strong absorption 
lines associated with cool dwarf stars, apparently indicating that they have a 
very bottom-heavy IMF (e.g. a power law with slope x~3). However, there is an 
important caveat to these conclusions: All of the most widely-used IMF 
indicators (the Wing-Ford band, Na I doublet, Ca II triplet) are also affected 
by the abundance of sodium, which has not yet been accurately constrained from 
independent features. Do giant ellipticals really have very bottom-heavy IMFs, 
or are they really enriched in sodium, to levels as high as [Na/Fe]~+1.0, as 
required to fit a Milky Way IMF? Here, we propose spectroscopy with FMOS in the
 1.1-1.4mum range, for a sample of 121 Coma Cluster galaxies, to measure new 
diagnostics which are sensitive to the IMF and to sodium independently. 
The same sample has been successfully observed with FMOS at shorter 
wavelengths. The proposed observations will break the IMF/sodium degeneracy 
and cleanly determine the variation of IMF with galaxy mass, metallicity and 
Mg/Fe ratio. [Re-application for cancelled 13A run]