Outer space for untwisted automorphisms of right-angled Artin groups

For a right-angled Artin group $A_\Gamma$, the untwisted outer automorphism group $U(A_\Gamma)$ is the subgroup of $Out(A_\Gamma)$ generated by all of the Laurence-Servatius generators except twists (where a {\em twist} is an automorphisms of the form $v\mapsto vw$ with $vw=wv$). We define a space $\Sigma_\Gamma$ on which $U(A_\Gamma)$ acts properly and prove that $\Sigma_\Gamma$ is contractible, providing a geometric model for $U(A_\Gamma)$ and its subgroups. We also propose a geometric model for all of $Out(A_\Gamma)$ defined by allowing more general markings and metrics on points of $\Sigma_\Gamma$.