The tautological ring of $\overline{\mathcal{M}}_{g,n}$ is rarely Gorenstein

TL;DR

This paper demonstrates that the tautological rings of moduli spaces of curves are not Gorenstein for certain genera and marked points, extending previous results and proposing a conjecture for the Gorenstein property in other cases.

Contribution

It extends the understanding of when tautological rings are Gorenstein, providing new non-Gorenstein cases and proposing a conjecture for the Gorenstein property based on genus and marked points.

Findings

Tautological rings are not Gorenstein for g ≥ 2 and 2g + n ≥ 24.

The proof involves intersection with non-tautological bielliptic cycles.

Several new cases where the tautological rings are Gorenstein are established.

Abstract

We prove that the tautological rings $\mathsf{R}^*(\overline{\mathcal{M}}_{g,n})$ and $\mathsf{RH}^*(\overline{\mathcal{M}}_{g,n})$ are not Gorenstein when $g\geq 2$ and $2g+n\geq 24$, extending results of Petersen and Tommasi in genus $2$. The proof uses the intersection of tautological classes with non-tautological bielliptic cycles. We conjecture the converse: the tautological rings should be Gorenstein when $g=0,1$ or $g\geq 2$ and $2g+n<24$. The conjecture is known for $g=0,1$ by work of Keel and Petersen, and we prove several new cases of this conjecture for $\mathsf{RH}^*(\overline{\mathcal{M}}_{g,n})$ when $g\geq 2$.