Planetary Nebulae as Dynamical Probes Planetary Nebulae as Dynamical Probes One of the outstanding problems in astronomy today is the question of the stellar kinematics and dark matter content of early-type galaxies. Most ellipticals possess smooth luminosity profiles, no emission features, and little neutral hydrogen, so the kinematic study of these objects is usually restricted to the absorption line measurements of their bright inner regions. The best way to resolve this problem is to identify test particles in the halos of these galaxies and measure their radial velocities.
Planetary nebulae are the perfect test particles.
So, in theory, PN radial velocity measurements should revolutionize our knowledge of the structure and evolution of early-type galaxies. In practice, however, attempts to measure PN radial velocities have met with limited success. The nearest large elliptical galaxies are 10 Mpc away, and PN measurements at this distance are just barely possible with 4-m class telescopes.