Amenable actions of discrete quantum groups on von Neumann algebras

We introduce the notion of Zimmer amenability for actions of discrete quantum groups on von Neumann algebras. We prove generalizations of several fundamental results of the theory in the noncommutative case. In particular, we give a characterization of Zimmer amenability of an action $\alpha:\Bbb G\curvearrowright N$ in terms of $\hat{\Bbb{G}}$-injectivity of the von Neumann algebra crossed product $N\ltimes_\alpha\Bbb G$. As an application we show that the actions of any discrete quantum group on its Poisson boundaries are always amenable.