Planetary nebulae are the perfect test particles. Hundreds of PN can be found in the outer envelopes and halos of a Virgo elliptical galaxies, and, because PN have narrow emission lines, virtually every object that can be imaged can also be measured spectroscopically to 5 km/s accuracy. 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. With the 8-m Hobby-Eberly Telescope the experiment will be easy!
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