MIGHTEE-H i: mass models and dark matter properties

Measuring galaxy rotation curves is critical for inferring the properties of dark-matter haloes in the Lambda Cold Dark Matter (\LambdaCDM) paradigm. We present HI rotation curves and mass models for 20 galaxies from the MIGHTEE survey. Using extended HI kinematics, we construct resolved mass models that include stellar, gaseous, and dark-matter components. Stellar masses are derived using 3.6 \mum imaging under fixed mass-to-light ratio (\Upsilon_{*} = M/L) assumptions and are complemented, for the first time for a HI-selected sample, by spatially resolved M/L, obtained from multi-wavelength SED fitting. We examine the ratio of baryonic to observed rotation velocity (V_{\rm bar}/V_{\rm obs}) at the characteristic radius R_{2.2}. Adopting a fixed \Upsilon_\star = 0.5\,M_\odot/L_\odot yields a clear dependence of V_{2.2}/V_{\rm obs} on galaxy luminosity, while adopting \Upsilon_\star = 0.2\,M_\odot/L_\odot substantially weakens this trend. In contrast, the resolved M/L analysis preserves the luminosity dependence while modifying the stellar contribution on a galaxy-by-galaxy basis, providing a more accurate representation of the underlying relation. We model the dark-matter haloes using Navarro-Frenk-White profiles and find that the different assumptions for a fixed a M/L systematically shift galaxies relative to the theoretical stellar-to-halo mass and baryonic-to-halo mass relations, while the spatially varying M/L yields the closest agreement with theoretical benchmarks within \LambdaCDM. We therefore demonstrate that future investigations of the dark matter properties of galaxies using rotation curves need to account for varying M/L across individual galaxy profiles and between galaxies in order to obtain accurate measurements of the dark matter, and therefore test \LambdaCDM.