A Combinatorial Characterization of Sol 3-Manifolds

We show that there is a universal compact branched 3-manifold $W$ such that a closed 3-manifold $M$ immerses into $W$ if and only if $M$ admits a Sol structure. Equivalently, a closed 3-manifold is Sol if and only if it has a certain type of triangulation. The construction of $W$ is based on a regular language that characterizes Sol manifolds.