Contracting the Weierstrass locus to a point

Abstract

We construct an open substack $U\subset\mathcal{M}_{g,1}$ with the complement of codimension $\ge 2$ and a morphism from $U$ to a weighted projective stack, which sends the Weierstrass locus $\mathcal{W}\cap U$ to a point, and maps $\mathcal{M}_{g,1}\setminus\mathcal{W}$ isomorphically to its image. The proof uses alternative birational models of $\mathcal{M}_{g,1}$ and $\mathcal{M}_{g,2}$ from arXiv:1509.07241.