Radial gas motions in The HI Nearby Galaxy Survey (THINGS)
  The study of 21cm line observations of atomic hydrogen allows detailed
insight into the kinematics of spiral galaxies. We use sensitive
high-resolution VLA data from The HI Nearby Galaxy Survey (THINGS) to search
for radial gas flows primarily in the outer parts (up to $3\times r_{25}$) of
ten nearby spiral galaxies. Inflows are expected to replenish the gas reservoir
and fuel star formation under the assumption that galaxies evolve approximately
in steady state. We carry out a detailed investigation of existing tilted ring
fitting schemes and discover systematics that can hamper their ability to
detect signatures of radial flows. We develop a new Fourier decomposition
scheme that fits for rotational and radial velocities and simultaneously
determines position angle and inclination as a function of radius. Using
synthetic velocity fields we show that our novel fitting scheme is less prone
to such systematic errors and that it is well suited to detect radial inflows
in disks. We apply our fitting scheme to ten THINGS galaxies and find clear
indications of, at least partly previously unidentified, radial gas flows, in
particular for NGC 2403 and NGC 3198 and to a lesser degree for NGC 7331, NGC
2903 and NGC 6946. The mass flow rates are of the same order but usually larger
than the star formation rates. At least for these galaxies a scenario in which
continuous mass accretion feeds star formation seems plausible. The other
galaxies show a more complicated picture with either no clear inflow, outward
motions or complex kinematic signatures.
