Extensions of finitely generated Veech groups

Abstract

Given a closed surface $S$ with finitely generated Veech group $G$ and its $\pi_1(S)$-extension $\Gamma$, there exists a hyperbolic space $\hat{E}$ on which $\Gamma$ acts isometrically and cocompactly. The space $\hat{E}$ is obtained by collapsing some regions of the surface bundle over the convex hull of the limit set of $G$. Using the nice action of $\Gamma$ on the hyperbolic space $\hat{E}$, it is shown that $\Gamma$ is hierarchically hyperbolic. These are generalizations of results from Dowdall-Durham-Leininger-Sisto, which assume in addition that $G$ is a lattice. Because finitely generated Veech groups are among the most basic examples of subgroups of mapping class groups which are expected to qualify as geometrically finite, this result is evidence for the development of a broader theory of geometric finiteness.