Dynamics of Barred Galaxies

Some 30% of disc galaxies have a pronounced central bar feature in the disc plane and many more have weaker features of a similar kind. Kinematic data indicate that the bar constitutes a major non-axisymmetric component of the mass distribution and that the bar pattern tumbles rapidly about the axis normal to the disc plane. The observed motions are consistent with material within the bar streaming along highly elongated orbits aligned with the rotating major axis. A barred galaxy may also contain a spheroidal bulge at its centre, spirals in the outer disc and, less commonly, other features such as a ring or lens. Mild asymmetries in both the light and kinematics are quite common. We review the main problems presented by these complicated dynamical systems and summarize the effort so far made towards their solution, emphasizing results which appear secure. Bars are probably formed through a global dynamical instability of a rotationally supported galactic disc. Studies of the orbital structure seem to indicate that most stars in the bar follow regular orbits but that a small fraction may be stochastic. Theoretical work on the three-dimensional structure of bars is in its infancy, but first results suggest that bars should be thicker in the third dimension than the disc from which they formed. Gas flow patterns within bars seem to be reasonably well understood, as are the conditions under which straight offset dust lanes are formed. However, no observation so far supports the widely held idea that the spiral arms are the driven response to the bar, while evidence accumulates that the spiral patterns are distinct dynamical features having a different pattern speed. Both the gaseous and stellar distributions are expected to evolve on a time-scale of many bar rotation periods. Submitted: July 1992, accepted November 1992, appeared February 1993 ii Sellwood and Wilkinson