Boundary maps, germs and quasi-regular representations

We investigate the tracial and ideal structures of $C^*$-algebras of quasi-regular representations of stabilizers of boundary actions. Our main tool is the notion of boundary maps, namely $\Gamma$-equivariant unital completely positive maps from $\Gamma$-$C^*$-algebras to $C(\partial_F\Gamma)$. For a unitary representation $\pi$ coming from the groupoid of germs of a boundary action, we show that there is a unique boundary map on $C^*_\pi(\Gamma)$. Consequently, we completely describe the tracial structure of $C^*_\pi(\Gamma)$, and under extra natural assumptions, we completely characterize the simplicity of the $C^*$-algebras $C^*_\pi(\Gamma)$. As an application, we show that the $C^*$-algebra generated by the quasi-regular representation $\lambda_{T/F}$ associated to Thompson's groups $F\leq T$ does not admit traces and is simple.