Universal Sub-threshold Substrate Equilibrium: Testing the UCM Isothermal Dark Matter Halo Profile against 176 Independent Galaxies from SPARC and LITTLE THINGS

Combined test of the UCM isothermal dark matter halo profile against SPARC (150 galaxies, Li et al. 2020) and LITTLE THINGS (26 dwarf irregulars, Oh et al. 2015) — two entirely independent HI surveys with different telescopes, reduction pipelines, and mass-modelling codes. Combined result: median χ² = 0.71 over 176 galaxies, 72% with χ² < 2. LITTLE THINGS dwarf galaxies fit marginally better (median χ² = 0.57), consistent with the UCM prediction that sub-threshold equilibrium is most accurate in the low-mass, feedback-free regime.