On algebraically closed fields with a distinguished subfield

This paper is concerned with the model-theoretic study of pairs (K, F) where K is an algebraically closed field and F is a distinguished subfield of K allowing extra structure. We study the basic model-theoretic properties of those pairs, such as quantifier elimination, model-completeness and saturated models. We also prove some preservation results of classification-theoretic notions such as stability, simplicity, NSOP₁, and NIP. As an application, we conclude that a PAC field is NSOP₁ iff its absolute Galois group is (as a profinite group).