Baryonic Deficits in SPARC Rotation Curves: Density- and Acceleration-Conditioned Trends in the Full Late-Type Sample

This paper introduces an empirical, density-gated extension to baryonic rotation-curve modelingthat reveals a previously unrecognized directional structure in the V_{\rm bar} / V_{\rm obs}diagnostic plane.Using the SPARC galaxy sample, we reproduce the observational framework of Starkman et al.(2018) and overlay physically normalized correction vectors predicted by Version 0.3 of aminimal density-regulated model. The model employs a mass-scaled transition radius and alocal stellar surface-density proxy, with only a single bounded nuisance parameter per galaxyand a fixed global normalization shared across the entire sample.The resulting directional response field shows a coherent, surface-brightness–ordered pattern:low-surface-brightness disks experience the largest regulated shift toward saturation nearV_{\rm bar} / V_{\rm obs} \simeq 1, while high-surface-brightness systems remain largelyunchanged. This behavior reproduces the classical maximal-to-submaximal disk sequencewithout invoking halo profile tuning, modified force laws, or acceleration thresholds.A single figure compresses what would otherwise require extensive rotation-curve fitting into afalsifiable geometric constraint: viable models must reproduce not only the observed distributionof galaxies in the V_{\rm bar} / V_{\rm obs} plane, but also the direction of regulated evolutionacross surface-brightness regimes.This work is intended as an empirical organizing law rather than a definitive physicalmechanism. The same mapping could arise from modified dynamics, baryon–halo coupling, oreffective medium responses, and cosmological interpretation is explicitly deferred.Reproducibility and transparency are emphasized throughout.A companion Zenodo notebook provides a copy-paste, human-readable replication guide usingSPARC-style rotation-curve data and is cited in this record. This work builds on the Density-Dependent Braneworld Borrowing (DDBB) framework introduced in Zapresko (2026), applying its density-gated organizing principles to galactic rotation-curve diagnostics. The present paper focuses on empirical structure and falsifiable directional predictions, without relying on the microscopic or cosmological interpretations of the original framework. https://doi.org/10.5281/zenodo.18296124