Invariant measures and traces on groupoid ${\mathrm{C}}^{\ast }$-algebras

We provide sufficient conditions for the existence of a trace on the essential ${\mathrm{C}}^{\ast }$-algebra of a (not necessarily Hausdorff) étale groupoid $G$ which extends an invariant measure $\mu$ on the unit space of $G$. In particular, it suffices for the isotropy groups of $G$ to be amenable, or for $G$ to be essentially free with respect to $\mu$. We also show that $G$ is essentially free with respect to an invariant measure $\mu$ if and only if $\mu$ extends to a unique trace on the full ${\mathrm{C}}^{\ast }$-algebra of $G$. We work in the generality of possibly infinite measures and, accordingly, possibly unbounded traces. Moreover, whenever possible, we state our results for twisted groupoids. As an application, we show that gauge-invariant algebras of finite-state self-similar groups admit a unique tracial state.