| Planetary Nebulae | Globular Clusters |
|---|---|
| The PNe allow us to address directly questions about the normal stellar population of a galaxy, such as its angular momentum at large radii, and orbital anisotropy -- data which provide key inputs into galaxy evolution models. | While the properties of the GC systems are interesting in their own right, shedding light on their formation, it is a questionable assumption to take their properties as representative of the stars in general at comparable radii. |
| PN kinematics at large radii can be tied directly to the stellar absorption line kinematics at small radii, thus providing a single dynamical tracer all through the galaxy. | There are typically not enough GCs in the inner parts of a galaxy to probe those regions, and combining the GC with the stellar constraints is not as powerful as if their kinematics were directly coupled. |
| The average spatial distribution of the PNe is quite well known from the integrated stellar light. | The GC spatial distribution is more uncertain, as it must be derived from discrete observations of the GCs themselves. This introduces larger uncertainties in any dynamical models. |
| Velocity measurements of PNe are very straightforward, as there is simply a single emission line at 5007Å to be measured, resulting in typical measurement uncertainties of ~20 km s-1. This advantage, and the previous two above, mean that a given number of PN velocity measurements will provide stronger dynamical constraints than a comparable number of GC velocity measurements. | Velocity measurements of GCs require careful use of stellar templates, resulting in typical measurement uncertainties of 50-100 km s-1. |
| The amount of telescope time required to attain a sufficent number of velocity measurements (~ 100-200) around a galaxy is roughly the same for PNe or GCs. However, lower-luminosity galaxies are typically more quickly studied with PNe, since the specific number of PNe per unit luminosity increases for fainter galaxies, while it decreases for GCs. | Galaxies like M87 and M49 with a large spatial extent are typically more quickly studied with GCs than with PNe using counter-dispersed imaging, since the latter would require taking multiple fields. |
| PNe can be found and have their velocities measured in a single observation, using counter-dispersed imaging. Starting from scratch, this gives a substantial savings in observing overhead. | Many elliptical galaxies already have large numbers of GCs identified through imaging surveys, meaning that the efficiency of follow-up spectroscopy is somewhat more predictable than for PNe, for which relatively fewer imaging surveys have been made. |
| PNe are observable in adequate numbers to typically ~5 effective radii. | Because of their more extended distribution, GCs may be observable to ~10 effective radii, thus better probing the largest radii. |