A Herbrand-Ribet theorem for function fields

Abstract

We prove a function field analogue of the Herbrand-Ribet theorem on cyclotomic number fields. The Herbrand-Ribet theorem can be interpreted as a result about cohomology with $\mu_p$-coefficients over the splitting field of $\mu_p$, and in our analogue both occurrences of $\mu_p$ are replaced with the $\mathfrak{p}$-torsion scheme of the Carlitz module for a prime $\mathfrak{p}$ in $\F_q[t]$.