The low degree cohomology of compactifications of $A_g$

TL;DR

This paper computes low degree $ ext{ell}$-adic intersection cohomology of symplectic local systems on the Satake compactification of $A_g$, revealing which Galois representations appear and characterizing Tate-type cohomology.

Contribution

It provides a complete classification of Galois representations in low degree cohomology of compactifications of $A_g$, independent of the compactification choice.

Findings

Only finitely many Galois representations appear in low degree cohomology.

Characterization of when cohomology is of Tate type.

Applications to holomorphic forms and cohomology of the interior.

Abstract

We compute the low degree $\ell$-adic intersection cohomology of symplectic local systems on the Satake compactification of the moduli space $A_g$ of principally polarized abelian varieties. We prove that only a small finite list of irreducible Galois representations can appear in the low degree cohomology of any nonsingular toroidal compactification of $A_g$ or $X_{g,s}$, the $s$-fold fiber product of the universal abelian variety. We give several applications, including to spaces of holomorphic forms on toroidal compactifications and to the cohomology of the interior. In particular, we give a complete characterization of when the cohomology of $X_{g,s}$, or one of its toroidal compactifications, is of Tate type. The result is independent of the choice of toroidal compactification.