MIGHTEE-H i: mass models and dark matter properties

Abstract

Measuring galaxy rotation curves is critical for inferring the properties of dark-matter haloes in the Lambda cold dark matter ($\Lambda$CDM) paradigm. We present H i rotation curves and mass models for 20 galaxies from the MIGHTEE survey. Using extended H i kinematics, we construct resolved mass models that include stellar, gaseous, and dark-matter components. Stellar masses are derived using 3.6 $\mu$m imaging under fixed mass-to-light ratio ($\Upsilon _{*} = M/L$) assumptions and are complemented, for the first time for a H i-selected sample, by spatially resolved $M/L$, obtained from multiwavelength spectral energy distribution 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 $\Lambda$CDM. 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 $\Lambda$CDM.