The Cosmic Structure Diagnostic Tool (CSDT): A Geometry-Based Structural Diagnostic for Rotation-Curve Features and Lensing-Peak Alignment

We present a geometry-only, model-free structural diagnostic (CSDT) applied to Spitzer IRAC 3.6 μm imaging of the interacting system NGC 6221/3256. The method quantifies the radial onset of lopsidedness in the stellar mass distribution without assuming equilibrium or parametric models. For NGC 6221, a sharp transition is detected at the outer disk (~260 px), while for NGC 3256 a clear geometric extent of the tidal debris field is identified at R ≈ 680 px (Spitzer scale). These results provide a complementary, morphology-driven check to radio continuum studies of interaction-driven disturbances.