Abelian varieties with many endomorphisms and their absolutely simple factors

Abstract

We characterize the abelian varieties arising as absolutely simple factors of $\mathrm{GL}_2$-type varieties over a number field $k$. In order to obtain this result, we study a wider class of abelian varieties: the $k$ varieties $A / k$ satisfying that $\operatorname{End}_k^0(A)$ is a maximal subfield of $\operatorname{End}_{\bar{k}}^0(A)$. We call them Ribet-Pyle varieties over $k$. We see that every Ribet-Pyle variety over $k$ is isogenous over $\bar{k}$ to a power of an abelian $k$-variety and, conversely, that every abelian $k$-variety occurs as the absolutely simple factor of some Ribet-Pyle variety over $k$. We deduce from this correspondence a precise description of the absolutely simple factors of the varieties over $k$ of $\mathrm{GL}_2$-type.