Five-dimensional compatible systems and the Tate conjecture for elliptic surfaces

Abstract: Let $(\rho_\lambda\colon G_{\mathbb Q}\to \operatorname{GL}5(\overline{E}\lambda))\lambda$ be a strictly compatible system of Galois representations such that no Hodge--Tate weight has multiplicity $5$. We show that if $\rho{\lambda_0}$ is irreducible for some $\lambda_0$, then $\rho_\lambda$ is irreducible for all but finitely many $\lambda$. More generally, if $(\rho_\lambda)\lambda$ is essentially self-dual, we show that either $\rho\lambda$ is irreducible for all but finitely many $\lambda$, or the compatible system $(\rho_\lambda)\lambda$ decomposes as a direct sum of lower-dimensional compatible systems. We apply our results to study the Tate conjecture for elliptic surfaces. For example, if $X_0\colon y^2 + (t+3)xy + y= x^3$, we prove the codimension one $\ell$-adic Tate conjecture for all but finitely many $\ell$, for all but finitely many general, degree $3$, genus $2$ branched multiplicative covers of $X_0$. To prove this result, we classify the elliptic surfaces into six families, and prove, using perverse sheaf theory and a result of Cadoret--Tamagawa, that if one surface in a family satisfies the Tate conjecture, then all but finitely many do. We then verify the Tate conjecture for one representative of each family by making our irreducibility result explicit: for the compatible system arising from the transcendental part of $H^2{\mathrm{et}}(X_{\overline{\mathbb Q}}, \mathbb{Q}_\ell(1))$ for a representative $X$, we formulate an algorithm that takes as input the characteristic polynomials of Frobenius, and terminates if and only if the compatible system is irreducible.