The Chow rings of the moduli spaces of curves of genus 7, 8, and 9

Abstract: The rational Chow ring of the moduli space $\mathcal{M}_g$ of curves of genus $g$ is known for $g \leq 6$. Here, we determine the rational Chow rings of $\mathcal{M}_7, \mathcal{M}_8,$ and $\mathcal{M}_9$ by showing they are tautological. One key ingredient is intersection theory on Hurwitz spaces of degree $4$ and $5$ covers of $\mathbb{P}^1$, as developed by the authors in [1]. The main focus of this paper is a detailed geometric analysis of special tetragonal and pentagonal covers whose associated vector bundles on $\mathbb{P}^1$ are so unbalanced that they fail to lie in the large open subset considered in [1]. In genus $9$, we use work of Mukai [23] to present the locus of hexagonal curves as a global quotient stack, and, using equivariant intersection theory, we show its Chow ring is generated by restrictions of tautological classes.