  We use 85 pairs of high resolution LCDM cosmological simulations from the
NIHAO project to investigate why in dwarf galaxies neutral hydrogen (HI)
linewidths measured at 50% of the peak flux W_50 /2 (from the hydrodynamical
simulations) tend to underpredict the maximum circular velocity VmaxDMO (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 HI
mass in both ALFALFA and HIPASS observations. 2) The HI 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 W_50 / W_20. Additionally, mass loss driven by
supernovae and projection effects (dwarf galaxies are in general not thin
disks) further reduce the linewidths. The implied HI linewidth velocity
function from NIHAO is in good agreement with observations in the nearby
Universe of dwarf galaxies: 10 < W_50 /2 < 80 km/s. 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.
