Low-weight compactly supported cohomology of $\mathcal M_{g,n}^{ct}$ and the extended Torelli morphism

Abstract

We study the weight zero and weight two graded pieces of the compactly supported cohomology of the moduli space $\mathcal M_{g,n}^{ct}$ of stable $n$-pointed curves of genus $g$ of compact type and show that they are computed by bridgeless graph complexes. In weight two, we construct a natural embedding of shifted weight zero cohomology from lower genus. For unmarked curves, we construct canonical classes represented by pinched odd wheel graphs and prove that distinct wheel monomials give linearly independent classes in the dual of $W_0H_c^{2g}(\mathcal M_g^{ct};\mathbb Q)$. By relating the bridgeless graph complex to the $K$-theory of graphs and the Quillen spectral sequence, we show that these classes remain linearly independent after pushforward along the extended Torelli morphism $\mathcal M_g^{ct}\to\mathcal A_g$. Finally, we prove that the realizations of the motivic Lie algebra arising from $\mathcal M_g$ and $\mathcal A_g$ have the same image in the weight zero compactly supported cohomology of $\mathcal M_g^{ct}$.