Characterisations and Galois conjugacy of generalised Paley maps

A generalised Paley map is a Cayley map for the additive group of a finite field F, with a subgroup S=−S of the multiplicative group as generating set, cyclically ordered by powers of a generator of S. We characterise these as the orientably regular maps with orientation-preserving automorphism group acting primitively and faithfully on the vertices; allowing a non-faithful primitive action yields certain cyclic coverings of these maps. We determine the fields of definition and the orbits of the absolute Galois group on these maps, and we show that if (q−1)/(p−1) divides |S|, where |F|=q=pe with p prime, then these maps are the only orientably regular embeddings of their underlying graphs; in particular this applies to the Paley graphs, where |S|=(q−1)/2 is even.