2.2. Do the PN Kinematics Follow the Absorption Line Stellar Kinematics?
If the distribution of PNe follows light, the PNe can be considered as faithful tracers also for the stellar kinematics. A test of this is to compare the rotation velocities v and velocity dispersions obtained from PN radial velocities with those from ALS, taking into account the greater smoothing of the PN velocity field due to spatial averaging. Mendez used his sample of 535 PN velocities in NGC 4697 for a simple comparison . His sample, restricted to a stripe around the major axis, indeed followed the rotation seen in the ALS data; however, the number of PNe per mean velocity point is ~ 10 and so the errors are large, ~ 50 km s-1. Similarly, the PN velocity dispersions in major axis sectors were consistent with the ALS dispersions within ~ 20 km s-1.
In such a case the PN velocities and the ALS velocity measurements can be simply combined in modelling the stellar kinematics. Even the incompleteness in the center does not present a problem, because the probability of losing a PN against the bright galaxy background is uncorrelated with its radial velocity. Then only the PN velocities, but not their radial distribution must be used in the modelling. If, on the other hand, because of an outward colour gradient and the observed dependence of B on colour, the distribution of PNe is more extended than the stellar light, say, then the PNe velocity dispersions would overestimate the stellar velocity dispersions, as can be seen most easily from the spherical Jeans equation. In this case, the PNe would have to be included in the modelling as a separate test particle distribution.
Another kinematic bias can occur for elliptical galaxies in galaxy clusters, in that the PN samples, particularly in the outer galactic halo, can be contaminated by intracluster PNe . ICPNe would be difficult to disentangle from galactic PNe when the velocity dispersions of the galaxy and its host cluster are comparable, such as for NGC 1399 in the Fornax cluster.