Quantifying Non-circular Streaming Motions in Disc Galaxies
  High-quality velocity maps of galaxies frequently exhibit signatures of
non-circular streaming motions. We here apply the software tool, "velfit"
recently proposed by Spekkens & Sellwood, to five representative galaxies from
the THINGS sample. We describe the strengths and weaknesses of the tool, and
show that it is both more powerful and yields results that are more easily
interpreted than the commonly used procedure. We demonstrate that it can
estimate the magnitudes of forced non-circular motions over a broad range of
bar strengths from a strongly barred galaxy, through cases of mild bar-like
distortions to placing bounds on the shapes of halos in galaxies having
extended rotation curves. We identify mild oval distortions in the inner parts
of two dwarf galaxies, NGC 2976 and NGC 7793, and show that the true strength
of the non-axisymmetric gas flow in the strongly barred galaxy NGC 2903 is
revealed more clearly in our fit to an optical Halpha map than to the neutral
hydrogen data. The method can also yield a direct estimate of the ellipticity
of a slowly-rotating potential distortion in the flat part of a rotation curve,
and we use our results to place tight bounds on the possible ellipticity of the
outer halos of NGC 3198 and NGC 2403.
