Absence of Realizable Subdominant Ringdown Modes in Gravitational-Wave Black Hole Mergers

Gravitational-wave ringdowns from binary black hole mergers are commonly modeled as superpositions of multiple quasi-normal modes, including subdominant modes and overtones. In this work, we perform a systematic empirical test of this assumption using publicly available strain data from the Gravitational Wave Open Science Center (GWOSC). We analyze ringdown signals from multiple high signal-to-noise gravitational-wave events using a hierarchy of increasingly permissive reconstruction strategies: unconstrained two-mode fits, numerically informed overtone models, and fully coherent multi-detector analyses enforcing shared frequencies and damping times across detectors. Coherent single-mode ringdowns are consistently recovered across detectors and events, validating data quality and reconstruction performance. In contrast, no stable realization of subdominant modes or overtones is obtained under any tested configuration, including fully coherent and noise-reducing fits. This systematic null result persists across all analyzed events and detector combinations. These findings indicate that currently observed gravitational-wave ringdowns are empirically consistent with a single, globally coherent post-merger mode, while independent subdominant ringdown structures are not observationally realized. Interpreted conservatively, the results place a falsifiable empirical constraint on models that predict independently observable internal resonances or reflective post-merger structures in black hole mergers.