Rotation-curve residuals reveal a suppressed acceleration branch in dwarf galaxies

Abstract

Galaxy rotation curves exhibit systematic deviations from the Newtonian expectation inferred from visible matter alone. Existing phenomenological descriptions capture many aspects of these deviations, but a common residual structure across massive disks and dwarf irregular galaxies remains unclear. We investigate whether rotation-curve residuals organize into a simple empirical form across the SPARC and LITTLE THINGS samples. We analyze 175 SPARC galaxies and 22 LITTLE THINGS dwarf irregular galaxies in velocity-squared space after subtracting a leading Newtonian-like term. We fit a generalized residual family, v^2-A/r=B+Cr^{q+1}, and examine which radial scaling is selected by the data. The galaxy population systematically favors the limit \(q\simeq0\), corresponding to an approximately linear residual relation, \(v^2-A/r=B+Cr\). SPARC galaxies generally occupy a high-\(B\) branch, whereas LITTLE THINGS dwarf galaxies show suppressed residual intercepts, including several systems consistent with \(B=0\). For the SPARC sample, the high-\(B\) branch approximately follows \(B\propto M_{\rm bar}^{0.72}\). {Rotation-curve residuals are not featureless scatter beyond the leading Newtonian-like contribution, but instead show a simple population-dependent empirical organization across massive and dwarf galaxy systems.}