Nonaxisymmetric models of galaxy velocity maps
  Galaxy velocity maps often show the typical pattern of a rotating disk,
consistent with the dynamical model where emitters rotate in circular orbits
around the galactic center. The simplest template used to fit these maps
consists in the rotating disk model (RDM) where the amplitude of circular
velocities is fixed by the observed velocity profile along the kinematic axis.
A more sophisticated template is the rotating tilted-ring model (RTRM) that
takes into account the presence of warps and allows a radius-dependent
orientation of the kinematic axis. In both cases, axisymmetry is assumed and
residuals between the observed and the model velocity fields are interpreted as
noncircular motions. We show that if a galaxy is not axisymmetric, there is an
intrinsic degeneracy between a rotational and a radial velocity field. We then
introduce a new galaxy template, the radial ellipse model (REM), that is not
axisymmetric and has a purely radial velocity field with an amplitude that is
correlated with the major axis of the ellipse. We show that best fits to the
observed two-dimensional velocity fields of 28 galaxies extracted from the
THINGS sample with both the REM and the RDM give residuals with similar
amplitudes, where the REM residuals trace nonradial motions. Best fits obtained
with the RTRM, because of its larger number of free parameters, give the
smallest residuals: however, we argue that this does not necessarily imply that
the RTRM gives the most accurate representation of a galaxy velocity field.
Instead, we show that this method is not able to disentangle between circular
and radial motions for the case of nonaxisymmetric systems. Finally, we
consider the physical motivation of the REM, and discuss how the interpretation
of galactic dynamics changes if one assumes that the main component of a galaxy
velocity field is modeled as a RDM/RTRM or as a REM.
