Toric geometry of $S{L}_2(\mathbb{C})$ free group character varieties from outer space

Abstract

Culler and Vogtmann defined a simplicial space $O(g)$ called outer space to study the outer automorphism group of the free group $F_g$. Using representation theoretic methods, we give an embedding of $O(g)$ into the analytification of $\mathcal{X}(F_g,S{L}_2(\mathbb{C})),$ the $S{L}_2(\mathbb{C})$ character variety of $F_g,$ reproving a result of Morgan and Shalen. Then we show that every point $v$ contained in a maximal cell of $O(g)$ defines a flat degeneration of $\mathcal{X}(F_g,S{L}_2(\mathbb{C}))$ to a toric variety $X(P_{\Gamma })$. We relate $\mathcal{X}(F_g,S{L}_2(\mathbb{C}))$ and $X(v)$ topologically by showing that there is a surjective, continuous, proper map ${\Xi }_v:\mathcal{X}(F_g,S{L}_2(\mathbb{C}))\to X(v)$. We then show that this map is a symplectomorphism on a dense, open subset of $\mathcal{X}(F_g,S{L}_2(\mathbb{C}))$ with respect to natural symplectic structures on $\mathcal{X}(F_g,S{L}_2(\mathbb{C}))$ and $X(v)$. In this way, we construct an integrable Hamiltonian system in $\mathcal{X}(F_g,S{L}_2(\mathbb{C}))$ for each point in a maximal cell of $O(g)$, and we show that each $v$ defines a topological decomposition of $\mathcal{X}(F_g,S{L}_2(\mathbb{C}))$ derived from the decomposition of $X(v)$ by its torus orbits. Finally, we show that the valuations coming from the closure of a maximal cell in $O(g)$ all arise as divisorial valuations built from an associated projective compactification of $\mathcal{X}(F_g,S{L}_2(\mathbb{C})).$

Added material on tropical geometry in Section 6

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