On the connected components of the moduli space of equivariant minimal surfaces in ${\mathbb{CH}}^2$

Abstract

An equivariant minimal surface in ${\mathbb{CH}}^n$ is a minimal map of the Poincaré disc into ${\mathbb{CH}}^n$ which intertwines two actions of the fundamental group of a closed surface $\Sigma$: a Fuchsian representation on the disc and an irreducible action by isometries on ${\mathbb{CH}}^n$. The moduli space of these can been studied by relating it to the nilpotent cone in each moduli space of $PU(n,1)$-Higgs bundles over the conformal surface corresponding to the map. By providing a necessary condition for points on this nilpotent cone to be smooth this article shows that away from the points corresponding to branched minimal immersions or $\pm$-holomorphic immersions the moduli space is smooth. The argument is easily adapted to show that for ${\mathbb{RH}}^n$ the full space of (unbranched) immersions is smooth. For ${\mathbb{CH}}^2$ we show the connected components of the moduli space of minimal immersions are indexed by the Toledo invariant and the Euler number of the normal bundle of the immersion. This is achieved by studying the limit points of the ${\mathbb{C}}^{\times }$-action on the nilpotent cone. It is shown that the limit points as $t\to 0$ lead only to branched minimal immersions or $\pm$-holomorphic immersions. In particular, the Euler number of the normal bundle can only jump by passing through branched minimal maps.

16 pages