The dynamics of barred spiral galaxies. I - The SBb galaxy NGC 7723

Spectroscopic observations of the SBb galaxy NGC 7723 are reported. The orientation of the galaxy in space and the nature of the velocity field in the bar region are discussed. The density-wave pattern speed in the galaxy is estimated on the basis of the angular velocity of the outermost H II region, assuming that this speed applies to both the spiral and the bar structure. A simple model for the potential in the bar region is outlined, gas orbits in the potential are calculated with the gas pressure neglected, and the strength of the nonaxisymmetric potential is estimated to be about 10% of the axisymmetric potential on the basis of dynamical arguments. It is found that the rotation in the bar region of NGC 7723 is approximately solid-body, while the spiral region is characterized by constant-velocity rotation, suggesting that solid-body rotation is a necessary, although perhaps not sufficient, condition for the bar phenomenon.