Accurate recovery of HI velocity dispersion from radio interferometers

Gas velocity dispersion measures the amount of disordered motions of a rotating disk. Accurate estimates of this parameter are of the utmost importance because it is directly linked to disk stability and star formation. A global measure of the gas velocity dispersion can be inferred from the width of the atomic hydrogen HI 21 cm line. We explore how several systematic effects involved in the production of HI cubes affect the estimate of HI velocity dispersion. We do so by comparing the HI velocity dispersion derived from different types of data cubes provided by The HI Nearby Galaxy Survey (THINGS). We find that residual-scaled cubes best recover the HI velocity dispersion, independent of the weighting scheme used and for a large range of signal-to-noise ratio. For HI observations where the dirty beam is substantially different from a Gaussian, the velocity dispersion values are overestimated unless the cubes are cleaned close to (e.g., ~1.5 times) the noise level.