MIGHTEE-HI: 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 ($Λ$CDM) 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 $μ$m imaging under fixed mass-to-light ratio ($Υ_{*} = M/L$) 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 $Υ_\star = 0.5\,M_\odot/L_\odot$ yields a clear dependence of $V_{2.2}/V_{\rm obs}$ on galaxy luminosity, while adopting $Υ_\star = 0.2\,M_\odot/L_\odot$ 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 $Λ$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 $Λ$CDM.

Figures (10)

• Figure 1: H I moment maps and position–velocity (PV) diagrams for two representative galaxies, J095720.6+015507 (top) and J100211.2+020118 (bottom). The left panel shows the H I moment-0 map, the central panel the moment-1 map, and the right panel the position–velocity (PV) diagram along the major axis, with the derived line-of-sight rotation curve (orange) and model (blue) overplotted. The synthesised beam is shown as an ellipse in the lower left corner.
• Figure 2: Rotation curves of our final sample colour-coded by the galaxies gas fraction F$_{g} =M_{\mathrm{HI}}/M_{\mathrm{bar}}$ (a) and by redshift (b). The measurement uncertainties are indicated by the shaded regions
• Figure 3: Inclination corrected H I surface mass density 1D profiles of our final sample, colour-coded by the galaxies' gas fraction F$_{g} =M_{\mathrm{HI}}/M_{\mathrm{bar}}$ (a) and by redshift (b). The measurement uncertainties are indicated by the shaded regions.
• Figure 4: IRAC 1 surface brightness profiles of our final sample colour-coded by the galaxies' gas fraction, $\mathrm{F_g} = M_{\mathrm{HI}} / M_{\mathrm{bar}}$ and normalized by the radius of the last measured ellipse (R$_{\rm max}$). The horizontal dashed line represents the Spitzer IRAC 1 $3\sigma$ magnitude limit
• Figure 5: Surface brightness and H I gas density profiles for two representative galaxies J095720.6+015507 and J100211.2+020118 from our sample. Left column: Profile for a galaxy with a pseudo bulge (orange points) described by a poly-exponential fit (orange line). The horizontal dashed line represents the 3$\sigma$ magnitude limit. The bottom panel displays the radial profile of the H I gas surface density (blue points), with the dashed blue line representing the poly-exponential gas disc fit and the points indicating the observed data. Right column: Profile for a galaxy without a bulge (orange points), described by exponential disc fit (line), the bottom panel shows the gas surface density profile (blue points) described by the poly-exponential gas disc fit (blue dashed line)