Mumford-Tate group and Galois representations

Question

Could someone please point me towards a proof of why the image of a Galois representation on the Tate-module of an abelian variety is limited by its Mumford-Tate group?

Answer 1

This is an immediate consequence of Theorem 2.11 and Proposition 2.9 in Deligne's 'Hodge cycles on abelian varieties' (notes by Milne available here: http://www.jmilne.org/math/Books/DMOS.pdf

2.11 shows that every Hodge cycle on an abelian variety $A$ over a field $k$ embeddable in $\mathbb{C}$ is absolutely Hodge, and 2.9 shows that all absolutely Hodge cycles are defined over a finite extension of $k$. So it follows that all Hodge cycles on an abelian variety have canonical $l$-adic realizations that are defined over the same finite extension of $k$. In particular, an open sub-group of the Galois group fixes all Hodge cycles; that is, it maps into the Mumford-Tate group of $A$.