5.1.6. Dynamics of Bars. V. Summary
A study is under way of absorption-line velocities and velocity dispersions in barred galaxies (Kormendy 1982b). The analysis is based largely on dimensionless parameters such as the local ratio of velocity to dispersion. This technique gives an overview of the internal dynamics without requiring a solution to the more difficult problem of constructing self-consistent models. Results obtained to date on the SB0 galaxy NGC 936 are as follows.
(1) The disk rotation curve is flat, as it is in SB galaxies with emission-line velocity data. Barred galaxies do not lack massive halos.
(2) Non-circular streaming motions are clearly detected. On the average, the stellar orbits are elongated parallel to the bar. A schematic description of the evidence and an illustration of the velocity field are given in Figure 43.
(3) The velocity dispersion in the bar region is 0.5 - 1.0 times as large as the maximum disk rotation rate. Large random motions and non-circular streaming are both present. Evidently a well-developed bar is a significant perturbation on the stellar dynamics.
(4) Two n-body bar models (Miller and Smith 1979, MS; Hohl and Zang 1979, HZ) have velocity fields quite similar to those observed. This includes agreement in qualitative features (cf. Figures 43 and 44) and in quantitative parameters (i) measuring the relative importance of ordered and random motions and (ii) the magnitude of "streaming" velocities in the reference frame of the bar (not illustrated). The bar of NGC 936 is only slightly hotter than the models.
(5) The structure and dynamics of the HZ and MS models are remarkably similar (see Kormendy 1982b for further details). Although this result may be expected from the similarity of their initial conditions, it is nevertheless a reassuring sign that comparisons with the observations yield fairly general results.
(6) The models have the following shortcomings. (i) Because they were formed dissipationlessly, they are much less flat than real bars. As a result, the z motions are not effectively decoupled from motions in the plane, which compromises comparison with the observations. (ii) No density discontinuity is seen at corotation or at any other radius. (However both models become less triaxial near corotation - Kormendy 1982b). (iii) The models are single-component systems. If, as in real galaxies, various components were formed at different times and with different amounts of dissipation, it is not clear how the results would be affected. E.g., even allowing for an extended period of bar formation, the bar in NGC 936 has been interacting with the other components for ~ 50 rotation periods.