Integrated Model of Stability in M31: Inner Braking, Resonance Coupling, and Observational Predictions

Abstract

We present a unified dynamical framework for the long-term stability of the Andromeda Galaxy (M31). We showthat M31’s high inner velocity dispersion is not merely a fossil of its merger history but an active stabilizing agentagainst bar-driven instabilities. We introduce a radius-dependent “Unified Stability Chain” consisting of: (i) innerbraking via a high-dispersion bulge (σR > 120 km s−1), (ii) a resonance locking mechanism where the bar’s OuterLindblad Resonance overlaps with the spiral Corotation Radius at R ∼ 9 kpc, and (iii) an outer kinematic plateau.Using tracer N-body simulations (N = 104) in a fixed potential, we show that this configuration maintains marginalstability (Qeff ∼ 1.1−1.5) over 1 Gyr. We provide falsifiable predictions: a ∼ 15 km s−1 radial anomaly detectable byGaia DR4 and a dispersion floor σR > 75 km s−1 testable with JWST/NIRSpec.