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 region s of disk spiral galaxies is considered. Two fundamentally different cases- noncircular motions in the triaxial bar potential and motions in circular o rbits but with orientation parameters different from those of the main disk -are shown to have similar observational manifestations in the line-of-sigh t velocity field of the gas. A reliable criterion is needed for the observa tional data to be properly interpreted. To find such a criterion, we analyz e two-dimensional nonlinear hydrodynamic models of gas motions in barred di sk galaxies. The gas line-of-sight velocity and surface brightness distribu tions in the plane of the sky are constructed for various inclinations of t he galactic plane to the line of sight and bar orientation angles. We show that using models of circular motions for inclinations i > 60 degrees to an alyze the velocity field can lead to the erroneous conclusions of a "tilted (polar) disk" at the galaxy center. However, it is possible to distinguish bars from tilted disks by comparing the mutual orientations of the photome tric and dynamical axes. As an example, we consider the velocity field of t he ionized gas in the galaxy NGC 972. (C) 2000 MAIK "Nauka/Interperiodica".