Nihao – XVII. The diversity of dwarf galaxy kinematics and implications for the H I velocity function

We use 85 pairs of high-resolution lambda cold dark matter (LCDM) cosmological simulations from the NIHAO project to investigate why in dwarf galaxies neutral hydrogen (H I) linewidths measured at 50 per cent of the peak flux W₅₀/2 (from the hydrodynamical simulations) tend to underpredict the maximum circular velocity V_max^DMO (from the corresponding dark matter only simulations). There are two main contributing processes. (1) Lower mass galaxies are less rotationally supported. This is confirmed observationally from the skewness of linewidths in bins of H I mass in both ALFALFA and HIPASS observations. (2) The H I distributions are less extended (relative to the dark matter halo) in dwarf galaxies. Coupled to the lower baryon-to-halo ratio results in rotation curves that are still rising at the last measured data point, in agreement with observations from SPARC. Combining these two effects, in both simulations and observations lower mass galaxies have, on average, lower W₅₀/W₂₀. Additionally, mass-loss driven by supernovae and projection effects (dwarf galaxies are in general not thin discs) further reduce the linewidths. The implied H I linewidth velocity function from NIHAO is in good agreement with observations in the nearby Universe of dwarf galaxies: 10 km s⁻¹ < W₅₀/2 < 80 km s⁻¹. The dark matter only slope of ≈−2.9 is reduced to ≈−1.0 in the hydro simulations. Future radio observations of unbiased samples with higher spatial resolution will enable stricter tests of the simulations, and thus of the LCDM model.