Complex Phase Structure of Kerr-AdS$_5$ Black Holes: Critical Saddles and Fisher Zeros

Abstract

We study the complex saddle structure of singly rotating Kerr--AdS$_5$ black holes using a two-variable reduced model in the grand canonical ensemble at fixed angular velocity, $Ω$. The small and large black hole saddles merge at a critical point where one Takagi singular value of the complex Hessian vanishes, identifying the soft mode associated with the merger. This merger remains distinct from the Hawking--Page transition throughout the physical domain $|Ω| < 1$. For the reduced integral with the standard measure, we show that thermal AdS dominates uniformly in a complex neighborhood of each real merger point when Newton's constant $G$ is sufficiently small. Consequently, this neighborhood contains no zeros of the partition function, even though the local saddle merger is described by Airy-type behavior. The Fisher zeros instead accumulate near the Hawking--Page coexistence line, with spacing of order $G$, through interference between thermal AdS and the large black hole saddle. We also show that the real quasi-Euclidean Kerr--AdS$_5$ family satisfies a strict asymptotic Kontsevich--Segal--Witten (KSW) phase bound for $|Ω| < 1$, with the merger point lying inside the allowed domain. In the fixed angular momentum extended ensemble, two such merger points combine into a cusp with a contour-dependent Pearcey approximation. These results distinguish local saddle degeneracies from global phase coexistence and clarify the different roles of Takagi modes, phase transitions, and zeros of the partition function in the complex black hole saddle structure.