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Galaxy velocity maps often show the typical pattern of a rotating disk,consistent with the dynamical model where emitters rotate in circular orbitsaround the galactic center. The simplest template used to fit these mapsconsists in the rotating disk model (RDM) where the amplitude of circularvelocities is fixed by the observed velocity profile along the kinematic axis.A more sophisticated template is the rotating tilted-ring model (RTRM) thattakes into account the presence of warps and allows a radius-dependentorientation of the kinematic axis. In both cases, axisymmetry is assumed andresiduals between the observed and the model velocity fields are interpreted asnoncircular motions. We show that if a galaxy is not axisymmetric, there is anintrinsic degeneracy between a rotational and a radial velocity field. We thenintroduce a new galaxy template, the radial ellipse model (REM), that is notaxisymmetric and has a purely radial velocity field with an amplitude that iscorrelated with the major axis of the ellipse. We show that best fits to theobserved two-dimensional velocity fields of 28 galaxies extracted from theTHINGS sample with both the REM and the RDM give residuals with similaramplitudes, where the REM residuals trace nonradial motions. Best fits obtainedwith the RTRM, because of its larger number of free parameters, give thesmallest residuals: however, we argue that this does not necessarily imply thatthe RTRM gives the most accurate representation of a galaxy velocity field.Instead, we show that this method is not able to disentangle between circularand radial motions for the case of nonaxisymmetric systems. Finally, weconsider the physical motivation of the REM, and discuss how the interpretationof galactic dynamics changes if one assumes that the main component of a galaxyvelocity field is modeled as a RDM/RTRM or as a REM.

Nonaxisymmetric models of galaxy velocity maps