Cohen--Macaulay Complexes, Duality Groups, and the dualizing module of Out (FN)

We explain how Cohen--Macaulay classifying spaces are ubiquitous among discrete groups that satisfy Bieri--Eckmann duality, and compare Bieri--Eckmann duality to duality results for Cohen--Macaulay complexes. We use this comparison to give a description of the dualizing module of Out (FN) in terms of the local cohomology cosheaf of the spine of Outer space.