SDSS IV MaNGA: Characterizing Non-Axisymmetric Motions in Galaxy
  Velocity Fields Using the Radon Transform
  We show how the Radon transform (defined as a series of line integrals
through an image at different orientations and offsets from the origin) can be
used as a simple, non-parametric tool to characterize galaxy velocity fields,
specifically their global kinematic position angles (PA_k) and any radial
variation or asymmetry in PA_k. This method is fast and easily automated,
making it particularly beneficial in an era where IFU and interferometric
surveys are yielding samples of thousands of galaxies. We demonstrate the Radon
transform by applying it to gas and stellar velocity fields from the first
~2800 galaxies of the SDSS-IV MaNGA IFU survey. We separately classify gas and
stellar velocity fields into five categories based on the shape of their radial
PA_k profiles. At least half of stellar velocity fields and two-thirds of gas
velocity fields are found to show detectable deviations from uniform coplanar
circular motion, although most of these variations are symmetric about the
center of the galaxy. The behavior of gas and stellar velocity fields is
largely independent, even when PA_k profiles for both components are measured
over the same radii. We present evidence that one class of symmetric PA_k
variations is likely associated with bars and/or oval distortions, while
another class is more consistent with warped disks. This analysis sets the
stage for more in-depth future studies which explore the origin of diverse
kinematic behavior in the galaxy population.
