The effect of projection on the observed gas velocity fields in barred galaxies
The problem of determining the pattern of gas motions in the central cases -- noncircular motions in the triaxial bar potential and motions in circular orbits but with orientation parameters different from those of the main disk -- are shown to have similar observational manifestations in the line-of-sight velocity field of the gas. A reliable criterion is needed fo the observational data to be properly interpreted. To find such a criterion, we analyze two-dimensional nonlinear hydrodynamics models of gas motions in barred disk galaxies. The gas line-of-sight velocity and surface brightness distributions in the plane of the sky are constructed for various inclinations of the galactic plane to the line-of-sight and bar orientation angles. We show that using models of circular motions for inclinations $i > 60^\circ$ to analyze the velocity field can lead to the erroneous conclusions of a ``tilted (polar) disk'' at the circumnuclear region. However, it is possible to distinguish bars from tilted disks by comparing the mutual orientations of the photometric and dynamical axes. As an example, we consider the velocity field of the ionized gas in the galaxy NGC 972.