On the automorphisms of Hassett's moduli spaces

TL;DR

This paper determines the automorphism groups of Hassett's moduli spaces of weighted stable curves, showing they are closely related to symmetric groups that preserve the weight data, especially in higher genus.

Contribution

It computes the automorphism groups of most Hassett's spaces in the Kapranov's blow-up construction and fully characterizes them for genus g ≥ 1.

Findings

Automorphism groups are subgroups of symmetric groups preserving weights.

For g ≥ 1, automorphism groups are explicitly computed.

In higher genus, automorphism groups are isomorphic to certain permutation groups.

Abstract

Let $\overline{\mathcal{M}}_{g,A[n]}$ be the moduli stack parametrizing weighted stable curves, and let $\overline{M}_{g,A[n]}$ be its coarse moduli space. These spaces have been introduced by B. Hassett, as compactifications of $\mathcal{M}_{g,n}$ and $M_{g,n}$ respectively, by assigning rational weights $A = (a_{1},...,a_{n})$, $0< a_{i} \leq 1$ to the markings. In particular, the classical Deligne-Mumford compactification arises for $a_1 = ... = a_n = 1$. In genus zero some of these spaces appear as intermediate steps of the blow-up construction of $\overline{M}_{0,n}$ developed by M. Kapranov, while in higher genus they may be related to the LMMP on $\overline{M}_{g,n}$. We compute the automorphism groups of most of the Hassett's spaces appearing in the Kapranov's blow-up construction. Furthermore, if $g\geq 1$ we compute the automorphism groups of all Hassett's spaces. In particular, we prove that if $g\geq 1$ and $2g-2+n\geq 3$ then the automorphism groups of both $\overline{\mathcal{M}}_{g,A[n]}$ and $\overline{M}_{g,A[n]}$ are isomorphic to a subgroup of $S_{n}$ whose elements are permutations preserving the weight data in a suitable sense.