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We prove that an outer action of a locally compact group $G$ on a full factor$M$ is automatically strictly outer, meaning that the relative commutant of $M$in the crossed product is trivial. If moreover the image of $G$ in the outerautomorphism group $\operatorname{Out} M$ is closed, we prove that the crossedproduct remains full. We obtain this result by proving that the inclusion of$M$ in the crossed product automatically has a spectral gap property. Suchresults had only been proven for actions of discrete groups and for actions ofcompact groups, by using quite different methods in both cases. Even for thecanonical Bogoljubov actions on free group factors or free Araki-Woods factors,these results are new.

Spectral gap and strict outerness for actions of locally compact groups on full factors