KESTEN'S THEOREM FOR INVARIANT RANDOM SUBGROUPS

An invariant random subgroup of the countable group Gamma is a random subgroup of Gamma whose distribution is invariant under conjugation by all elements of Gamma. We prove that for a nonamenable invariant random subgroup H, the spectral radius of every finitely supported random walk on Gamma is strictly less than the spectral radius of the corresponding random walk on Gamma/H. This generalizes a result of Kesten who proved this for normal subgroups. As a byproduct, we show that, for a Cayley graph G of a linear group with no amenable normal subgroups, any sequence of finite quotients of G that spectrally approximates G converges to G in Benjamini-Schramm convergence. In particular, this implies that infinite sequences of finite d-regular Ramanujan-Schreier graphs have essentially large girth.