On the Kodaira dimension of the moduli space of nodal curves

Abstract

We show that the compactification of the moduli space of $n-$nodal curves of genus g, i.e. $\mathcal{N}_{g,n}:= \mathcal{M}_{g,2n} /G$, with $G:=(\mathbb{Z}_2)^n \rtimes S_n$, is of general type for $g \geq 24$, for all $n \in \mathbb{N}$. While this is a fairly easy result, it requires completely different techniques to extend it to low genus $5 \leq g \leq 23$. Here we need that the number of nodes varies in a band $n_{\mathrm{min}}(g) \leq n \leq n_{\mathrm{max}}(g)$, where $n_{\mathrm{max}}(g)$ is the largest integer smaller than (or in some cases equal to) $\frac{7}{2}(g-1)-3$. The lower bound $n_{\mathrm{min}}(g) $ is close to the bound found by Logan and Farkas for $\mathcal{M}_{g,2n}$ to be of general type (in many cases it is identical). This will be tabled in Theorem 1.1 which is the main result of this paper.