Cusps, cores, and one acceleration: dark-matter haloes versus modified dynamics across the full SPARC sample

Abstract

The rotation curves of the 175 disk galaxies in the SPARC database are confronted, one by one and under identical conditions, with the two mass models that have dominated the missing-mass debate for four decades: a cold dark-matter halo of Navarro-Frenk-White (NFW) form, and Milgromian dynamics (MOND) with the acceleration constant held at $a_0=1.2\times10^{-10}$ m s$^{-2}$. Stellar mass-to-light ratios, distances and inclinations are treated as nuisance parameters with priors set by the 3.6 $μ$m photometry and by the SPARC error budget, and every galaxy is sampled with a Markov-chain Monte Carlo. Three findings emerge. (i) Judged by the Bayesian information criterion, the free NFW halo is preferred in 96 galaxies and MOND in 79; once the halo is required to obey the $Λ$CDM concentration-mass relation the score becomes 86 to 89, i.e. a draw. The two theories fail in different places: MOND loses in massive, high-surface-brightness, bulge-dominated systems, NFW loses in slowly rising dwarf rotation curves. (ii) The NFW concentrations demanded by the data sit a median 0.24 dex below the $Λ$CDM expectation, and 53% of the sample lies more than $2σ$ below it, independent of halo mass; this is the core-cusp problem expressed as a scaling-relation offset rather than as a handful of anecdotes. (iii) The radial acceleration relation built from the full sample has $a_0=(1.13\pm0.02)\times10^{-10}$ m s$^{-2}$ and an observed scatter of 0.14 dex, yet the value of $a_0$ preferred by individual galaxies spans more than a decade and correlates with surface brightness. The two statements are not in contradiction: the population-level relation is tight because it averages over exactly the degeneracies with mass-to-light ratio and distance that make $a_0$ ill-determined galaxy by galaxy. The full table of fits is provided.