On the non-abelian Hodge locus I

Abstract: We partially resolve conjectures of Deligne and Simpson concerning $\mathbb Z$-local systems on quasi-projective varieties that underlie a polarized variation of Hodge structure. For local systems of "compact type", we prove (1) a relative form of Deligne's finiteness theorem, for any family of quasi-projective varieties, and (2) algebraicity of the corresponding non-abelian Hodge locus.