  For n greater or equal 4 we discuss questions concerning global fixed points
for isometric actions of Aut(Fn), the automorphism group of a free group of
rank n, on complete CAT(0) spaces. We prove that whenever Aut(Fn) acts by
isometries on complete d-dimensional CAT(0) space with d is less than 2 times
the integer function of n over 4 and minus 1, then it must fix a point. This
property has implications for irreducible representations of Aut(Fn), which are
also presented here. For SAut(Fn), the unique subgroup of index two in Aut(Fn),
we obtain similar results.
